Hi, if my rear stay is not disc specific, can I still use a rear wheel that has a disc hub?

Sure, just leave off the un bought disc, there is sufficient room on 135 wide MTB hubs as they are,

to add the extra metal to machine for the mounting .

make sure the rim is rim brake compatible

Aren't they all?

How do I tell?

No they aren't. There are a lot of disc only rims that are rounded quite sharply where the rim brake track should be. You can't use those rims on a rim brake bike. So look at the pictures to ensure that the rim has a nice flat section right at the tire bead. Ideally you'd search for the rim's maker and model and it'll tell you in the write up or specs or by the look of the cross section diagram if the rim will work with rim brakes.

It used to be that mountain rims were mostly rim brake compatible. But looking at the major makers they are offering only half or less of their total MTB lineup that are rim brake compatible now.

No, not any more. There are now rims made with no specific brake tracks and are intended for use only with disc brakes. Look at the rim and see if there are obvious brake tracks, particularly machined ones. If the rim sidewalls are smoothly curved with no apparent flat faces for the brake shoes to ride on it's intended for disc brakes only.

It's not all that obvious on some rims that they're not intended for rim brakes, at least regarding the shape of the rim. This rim has what might look like a brake surface at first glance, but the area that would be the brake surface is really too narrow for a brake pad.

Interesting. Does Mavic specifically state that rim is disc brake only?

Yes, at least to the extent that they call it a disc rim and the Mavic mtb rims and wheels that are rim brake compatible have this compatibility listed in their description. And of course this rim is called "TN 719 Disc." I don't think it's obvious in my picture, but the area on the rim that looks like it might work as a brake surface is really too narrow, a brake pad would overhang it a bit.