It seems like the multibrush tool can only be rotated by an integer value, which I'm not finding precise enough for my current project (based on what I've already drawn, an angle of about 38.4 degrees would let me continue as I had intended, but neither 38 nor 39 is close enough).

How difficult would it be to change this?

That probably isn't going to be too hard to change that to a decimal. It looks like internally it is already a decimal value. The UI only allows for integer values. This change should just be updating the UI control so it sens decimal values instead of integer values.

connect (customUI->axisRotationSpinbox, SIGNAL(valueChanged(int), this, SLOT(slotSetAxesAngle(int))

the angle that is being set is a qreal type which has much more precision.

I just updated this. It will be in Krita 4.1, or the daily builds. You can get the daily builds from the download page on krita.org

You can specify decimal places on the multibrush rotation angle

Many thanks!