Update:

Jeremy Murphy has informed me that v1.8 of simplicity (so in other words taskbar3.dll v0.303) works fine.

Kim Hoang reports that v1.6 of simplicity does indeed work fine after all, but the taskbar in v1.8 still has problems.

(As I said above though, I think I'll just go with v0.303 on 9x - sorry Kim - I'm hoping that will be good enough.)