Okay westfw and pito, after a nights sleep, I am willing to add another mounting hole for a smaller coin cell battery holder, wired in parallel with the existing, if you can help me with a symbol for it.
You can see space is a little tight in that area. Options are to use the existing Bat+ hole and add a new Bat- hole, or use the existing Bat- hole and add a new Bat+ hole.
Ah! I see it is surface mount - that will be easier to add, there are not too many bottom traces in that area.
Rats! I went thru a lot of effort to make sure no thru parts would interfere with the battery holder - then stuck a thru hole +5V regulator right in the middle of it! Back to the drawing board - or, forming the TO-220 leads. Really wanted thru hole for mechanical strength tho to support addition of a heat sink.