binoscope questions

I am considering constructing a bino-scope using a couple of 90mm objectives coupled to two halves of an old pair of BAK4 binoculars and 1.25" eyepieces.

I'm wondering though, how high could I expect to go with magnification before the exit pupil size makes them too inconvenient to look through? (I have dreams of mounting them on an equatorial fork for planetary observations.)

Anybody ever tried that before?

Thanks.
------------Steve.