Freewheel has some support to force scrolling on MUI toolbars by IDCMP events (mouse scrolling).
I just tried and here it works. as soon as i run FreeWheel scrolling works in iBrowse.
Note that i'm using a USB Mouse over Poseidon + the v50 input.device from Ralph Schmidt.
However this should work with any FreeWheel compatible mouse driver (Cocolino/PS2m have them).