Have you tried putting the input mask at the "COLUMN" level... Not the textbox level. The textbox takes priority if it has focus. The COLUMN level is for ALL rows regardless of current or not.

I even tested this from VFP5. I used a table with currency, then did

BROWSE NAME TEST1

then in the command window changed mask via...

Test1.Column1.Inputmask = "$$,$$$.99" and it worked for ALL rows even when grid didn't have focus at all.