It seems all but certain that the byte at 0x0140, known to have an effect on the amount of faces that can be rendered, is actually part of a two-byte value (0x013F-0x0140). It in turn seems that the range of accepted values here are 0x000F to 0x044C. All values outside this range are identical, but I'm not sure exactly how it's calculated - I thought it was "always 0x0300, or something close to it", but that might just be the cap rather than an exact value, due to what I've observed re: replay breakage (see below).
EDIT: Values above 0x0439 reduce the maximum number of lemmings you can have in the level (actually, even 0x0439 and a bit below probably does, but since you shouldn't have more than 80, and 80 lemmings works with a value of 0x0439...). At 0x044C, the limit is 65 lemmings.
It also seems at a glance - though I didn't check this too in-depth - that the rule "0x013E = 0x0140" in official levels, extends to this as a two-byte value, ie: in official levels, the two bytes 0x013D-0x013E, always have the same value as the two bytes 0x013F-0x0140. I still have not found any effect of changing 0x013D and/or 0x013E in a level, though.
I've also discovered, that replays working seems to depend on whether the value is above or below 0x02AC. Take this with a grain of salt, as this is based on testing a single level with a single replay. If a replay is recorded on a level where the value is 0x02AC or above, then the level is modified to make this value below 0x02AC (or vice versa), the replay breaks. This explains the replay breakage I saw with L3DEdit - all were in cases where I modified a level created with an old version of L3DEdit, using a newer version. Older versions left 0x0000 in these bytes, while newer ones write 0x0400 (which will be changed to 0x044C 0x0439 in the future, if I don't find any replay breakage or other negative side effects of such a change). EDIT: Based on further testing, there seems to be nothing special about 0x02AC outside of the context of that particular level. I've seen breakage on a different level when changing from 0x0400 to 0x0439. Assume any change to this value can break replays.