Thanks for the replies and advice! In reply . . . .
The IR remote idea is certainly interesting. I had previously considered a wired remote, but it seemed unworkable. Saying that, rewiring one with a sealed electrical button/switch certainly has some appeal. I'll keep the IR remote in mind as my back-up plan, but the design I had in mind for the case would probably shroud or cover the IR sensor on the front of the camera and I wasn't aware of a sensor on the back of the K-01? My bad if there is.
The DIY camera housings that the surfer makes are pretty phenomenal! His button design wouldn't be too difficult to make, either.
I had previously browsed some sites that sell housings and parts, but the sampling I took of local Australian-based websites didn't seem to have anything other than basic accessories for sale (lenses, strobes, arms, etc.). I've had another look ( and will continue to do so ), and am now looking at another option that appeals. Instead of a sprung button, it looks like many of the higher quality Ikelite ( and other ) cases have a sealed lever that activates the shutter button. A spring wouldn't be necessary, there are fewer critical sealing surfaces for the O-rings to seal on than for a button/plunger, and it would probably feel a lot more natural to operate. Possibly a little easier to manufacture than a button as well.
Ah; decisions, decisions. I can't promise I'll even make a start within the next 6 months, as I'm a chronic procrastinator; however if I do build one I certainly will make a dedicated post for the interested folk.
And yes, the K-01 was originally intended to be just my back-up camera for my K-5; underwater adventures weren't even in mind when I bought it! It's also an easier shape to make a box to! My flatmate is also a keen photographer ( probably more so than I: he has deeper pockets ), and he's just started playing with a custom housing for his D5300 ( or does it suit his D750? I can't recall ) . . . Psychoanalyze my response to that as much as you like, but I think it'll be fun to go with him on some snorkels/dives whilst rocking the K-01