Hello again folks!
In the end - or, thus far - I decided to do nothing. For the last few open mics (as a guest player) I rolled out my modified Crate BT50. I put casters on it cos it's 20 Kg, as well as an Eminence driver in it which gives me better bottom end for the same size box. That's worked well so far, except that it wont compete with a moderately loud outfit, incl drummer... which is totally expected, of course. On all these occasions I've used my KALA UBass. I will try my upright through it and an electro-acoustic BG and/or an electric BG.
Another option is that my main rig is the Genz Benz Streamliner 600 + 2, 1x12 cabs. I suspect the head + 1 cab might work well for the situations mentioned in the opening post. Certainly lighter and easier to carry/move about than the Crate and should pack a better punch when the noisy drummers step up to the stage!!
It may turn out that I have enough gear after all. I can afford any kit out there on the market, but there's simply no point, IMHO, to buying more that effectively duplicates what I already have.
I will let time and experience take it's natural course ...
Thanks to all contributors.
Best.
j