I'm considering buying a steel cross frame, but hesitant because the rear will only accept a true 30mm wide tire. (I won't be racing, not concerned with UCI rules)

The seller tells me that there is ample clearance at the rear of the seat tube and between the seatstays to allow for much wider tires.

Would it be possible (without framebuilding/specialty tools---I just have basic household stuff) to dimple each chainstay about 2-3mm? I'm not too concerned with damaging the paint on the inside of the stays but don't want to do any structural damage or screw up the alignment.

If possible, any suggestions for how I should go about it?

you put a shaped wooden block in a vise with the frame to form the dimple. On the outside, you use a wooden block with a channel cut in it the shape of the chainstay

You can always bet that Alex Wetmore has done whatever you want to do, see this post

Is that the frame? It has carbon stays, you can't dimple them. It does look ample though.

Chainstays are steel. I know I can't dimple carbon. Clearance is not ample.

A wooden pair of crimping forms clamped with cee clamps one stay at a time. Lots of hand forming of the wood blocks to make the results look smooth. Far easier done before the stays are brazed in place... Andy.