I think it would not be a huge problem to build this in as an option, and it would make XMPlay one step more portable than others...
xmplay.exe -settings "path/to/xmplay.ini and other stuff"
Why would you want to use multiple configurations on the same portable device?
Is there some documentation on other commandline functions?
- -tray: start XMPlay in tray
- -play: starts playing directly (useful when no file or playlist is explicitly opened)
- -boost: increase GUI priority
That''s about it

.
And is it possible to for example add the secret ini-options by adding a commandline parameter?
No. Shouldn't be that hard to do that without starting XMPlay.