I tested my Holux with rechargeable batteries from Ansmann and can report that they work fine (even on a winter day in the mountains). I’ve used the following type: Ansmann maxE 2100 mAh (it’s a NiMH HR6 with 1.2 V and 2100 mAh)
One problem I noticed though was that the battery diameter is too big meaning that you can get it in but not out again (at least not easily). Removing the wrapper helps. I don’t know whether the battery compartment is too narrow or the battery itself too wide. According to the AA specs, the diameter is supposed to be 14.0 +/- 0.5 mm.
NiMH batteries also have a tendency to expand their size after few charge-recharge cycles. You are very correct about the size given in m241 is limited. Experience with duracell alkaline batteries is great cause they are within limits, also, NICD are also easy and very right.
By the way, which year did you buy your m241. In their last edition they have changed the circuits to only allow alkaline. rechargeable are discouraged on their site, I find NiMH batteries don’t work even below 1.3V charge. Earlier editions of m241 even drained it to 1.15V which means many more hours of uptime.
I’ve bought mine about two years ago. I assume mine is the original design which works with NiMH.