maybe a stupid idea, i dunno, but what about drawing the gui through opengl, it could be then easy resized in windowed mode with nice filtering…
some emulators like winuUAE do this and it works nicely…
you could also implement scanlines for the true oldschool feeling 
it can be easily done using glDrapPixels(). but it isnt THAT simple in case of renoise. its build ontop of directX. switching to antother graphic-context is possible, but wont happen until 2.0 IMHO. in renoise 2.0 we will be targeting full modularity so ports to another platforms can be easily done. right now we are implementing many abstraction layers due the mac-port, so many of those changes can be reused in RNS2.0.
until then, you will need to paint some stripes on your monitor 