After talking with the coders, there is a way to do this. It's a bit hacky, but if you are careful it should work.
If you open your .rba file in a text editor (it's probably too big for notepad, but something like EditPlus) you will see a well formed .dta file near the top of the file.
If you search for 'tuning_offset_cents' you can edit the value from +50 to -50. Each cent is 1/100 of a half step, so you have enough control to adjust the tuning of your song up or down to the nearest "real" note, if that makes sense.
You will probably have to make this edit manually every time you recompile, as Magma will stomp the value to 0.00 (we haven't tried this, so let us know). Also, make sure that you open and edit this in a decent text editor that doesn't try to save the file at the end as .txt, or add special curly braces, or whatnot. That will nuke your .rba file and ruin your day.