I know that you have some 'persons of size' out in the States, but I
have never flown a glider that could not stalled in straight flight

Not that this adds to the main discussion, but...

160lb (normal sized) pilot here... my SZD-59 couldn't be stalled
straight ahead without 'tricking' it (ease nose up till *near stall
and yank the stick for the last bit of travel). No mods were done to
plane to cause this, and a reliable W&B with me in it determined C/G
to be at 37%, well within the limits (it's JAR22 cert'd...). This is
actually common with this ship as well as the Jantar Std3 from which
it's derived. I added 5.5lbs to the tail via a brass/lead tailwheel
and now fly it at 50%, where it flies much nicer, with an honest (and
predictable) stall and a more usable trim range. At some point I'm
going to make an externally visible, removable weight set (replacement
rudder hinge access panels milled out of brass or tungsten) to add up
to 3 more lbs, to get it to 65% for mid-late season flying but plan on
leaving it at 50% for early season.

-p