do you think your 'proper' shruti has enough puff to blow a melodica?

Some I've seen just might, the 'four dial' type maybe, but they tend to use pretty soft reeds some of which blow under the weight of gravity on the reciprocal bellows - so maybe some sort of wind-chest would give a better pressure. Mind you, I'm thinking of the Hohner button melodica-alto which takes a lot of puff!

Rachel's new lap-top style harmonium has a reciprocal (sprung) bellows arrangement which (once we've sealed the gaps) works a treat on an stupidly loud set of reeds for the size of the instrument, so it could work. Our other Indian harmonium (baja) uses a horizontal bellows + wind-chest, which makes more sense to me & gives a more consistent pressure.

You got a WASP!? Bleedin' hell. Robert Wyatt used one of these on The Animals Film soundtrack & it even gets name-checked by Mark E. Smith on the 1983 live version The Fall's Mere Psued Mag Ed.:

Has a sneer which was weird
Some time ago
Heard Kraftwerk in '81
Has a WASP synthesizer

A real male, make-up as well
Sophisticate
WASP synthesizer
Didn't get far
In computer teaching job
His dream girl sings adverts for Renoir perfume
A fancied wit that's mere imitation of D. Bowie
in "Man Who Fell to Earth"

I tell you, these things are serious cult artefacts; the idea of your lad playing one & drumming at the same time makes me very happy indeed. The stylophone is seriously cult too of course - at least you buy these new. I use one in conjunction with various loops & filters - see the track Stylophony Number One playing HERE.