Playing in a parade is certainly compatible with a respectable musical performance, marching bands throughout the USA do this at every high school and university football game, and on several holidays. (admittedly some of those performances are more worthwhile than others.)
I played Alto Horn in my high school marching band many years ago, and more recently played in a street samba band. a few years ago. I've not tried playing my Anglo in a parade, but I know there are a few things to keep in mind.
The obvious one you have already identified and that is the difference between playing while sitting, and playing while standing. For me it isn't an issue, because I normally don't rest either end or any part of the bellows on my knees or lap while playing. Yet I no longer keep the straps very tight, although I admit my hands are probably larger than yours. So maybe that is a place to start. While still sittng, try playing with the instrument completely off your lap. That probably means pointing your forearms upward somewhat above level, but it doesn't necessarily mean pointing them at a steep vertical angle, which would feel awkward. I should also admit that I primarily play a 20 B wooden ended Anglo, and I do appreciate the light weight.
Next consider that it is tiring if the parade is very long. Consider the route of the parade; just how often do you take a brisk walk over that distance while lifting an object the size of your instrument? And playing for an extended period is tiring too. Of course participating in the event is exciting, so that helps. But it is a good idea to try walking a bit with your instrument as a method of preparation, so you know what you are getting into. It does take some upper body strength to avoid letting your arms drop toward the end, and fingers get clumsy when arms are tired.
Next, consider whether you will be walking freely, or marching in time to the music. After those years in marching band, and many years of dancing since, I find it easier to step in time to the music than to just walk, but that certainly wasn't the case at first. So if you are marching in time, you need to practice that as well, and consider what pace those steps will need to be for the various tunes you expect to play. Some tunes just aren't paced for marching, and you may not want to take a step on every beat of a fast jig!
If after trying it for while, lifting the instrument free hand seems like too much, a neck strap could be fastened to secure webbing loops tightly fastened around each of the ends. Wrapping the webbing around the ends could eliminate the need to screw something into the instrument, or unduly stress an exising strap screw. The neck strap would then support the ends at whatever height you find comfortable, while still allowing lateral movement for playing. It would put stress on your neck of course, and I don't think I would like wearing it, but it might be a solution for you. Again, try it out thoroughly before the day of the parade, and not just while standing still. I've seen more than a few last minute solutions fall apart halfway through a procession. Same goes for any type of platform. I really wouldn't expect that to survive.
That left hand thumb under the strap idea might work too. I also saw a thread here some months ago where someone was doing that with both thumbs because they hadn't seen the usual hand position, but once the difference was pointed out, they still seemed to prefer it.
Your concertina shouldn't suffer any harm as long as:
a - You don't drop it. Remember that hands get clumsy when tired, and arms tend to droop downward over time. But if you choose to use a neck strap, don't trust that with the full weight of the instrument either, but just use it to assist your arms in holding the concertina up. If you take your hands away from the instrument and let it dangle you can't feel when it starts to slip.
b - You prepare in case of rain. If it is raining, don't take the concertina. But even if it isn't supposed to rain, remember that you won't be able to get to shelter immediately if the weather shifts, and the length of time you will be at the event is much longer than the time while in the parade. It usually involves lining up a while beforehand, and likely involves standing around for speeches and celebrations after, and almost certainly means walking back to where you started as well, so keep a sturdy plastic bag in your pocket, just in case. Rain damage wasn't an issue when marching with a brass instrument like the horn, but that horn got rained on many times. ( I see you are in Tucson - well, it could happen. I've not been to Arizona yet, but I've been rained on in summer in New Mexico and Colorado a number of times. Of course If it doesn't rain you should take water for yourself though, as you must already know if you live there!)
I hope I haven't made the whole process sound too daunting. Playing in a parade is great fun, and I hope you get to enjoy it!
Edited by Tradewinds Ted, 14 August 2014 - 03:40 PM.