As I recall, US FAR Part 23 and the very similar JAR/CS 22 specify that a normal category glider be able to accommodate a 100 lbf chordwise force at the wingtip unless lower can be rationally justified. My impression is that most gliders are certificated to lower tip forces, the rational justification being "we handle them with care."

I looked it up. JAR 22.501 Wing-tip landing

"There must be means to ensure that ground
loads acting at the wing tips are adequately
resisted. A limit load T = 40 daN must be
assumed to act rearward at the point of contact of
one wing-tip with the ground, in a direction
parallel to the longitudinal axis of the sailplane,
the yawing moment so generated must be
balanced by side load R at the tail skid/wheel or
nose skid/wheel (see Figure 4)."

40 daN of force at the wingtip works out to about 89.9 pounds-force. That's pretty close to your recollection of 100 lbf. I don't know what your average wing-walker can generate in terms of pushing force. (I can't say I've ever placed a bathroom scale on the wall and pushed on it with all my strength). I also don't know what kind of standards were required for the older antique gliders; they may have been certified before JAR-22 was even a thing.

I still don't let people help push my Duo from the wingtip. Come in-board to the spoilers if you want to push.