After reading the last few posts, I thought about this again, and maybe changed my mind (now knowing some new thoughts about this)!
resetting the score is the main difference from "no time limit". the idea came from my playing expirience: i am not a good player, but i love to "compete with myself". usually i prefer to explore the level without paying attention to time limit first, and later try to beat it with the time limit turned on. but i am too lazy to constantly switch the option.

i am also rarely playing the whole level set at once, it is more like "play some levels from one set, then some from another, then maybe continue the first one". so if i want to play some new levels, then return to old ones to beat them "in time", i need to go through the options time and time again. and "half-failed" levels (finished, but not in the given time limit) still pollute the highscore table, which lowers my desire to play the level again.
Well, I totally understand that it makes things very confusing if you cannot see clearly which scores in the highscore table are "valid" and which are "invalid" (caused by levels played with "time limit: off"). That all makes sense to me!
On the other hand (having filbo's last suggestion with three possible "time limit" options in mind), I really would like to keep things as easy (and therefore user-friendly, hopefully) as possible!
So what about the following approach: When playing with "time limit: off", the program could be changed to effectively behave like you suggested for the new "explorer" mode, that is: As soon as the time runs out, the score is set to "0", so the local high score list won't be polluted with "invalid" entries (that look valid with a non-zero score, but won't be valid tapes to solve the level when applying the regular time limit). Additionally, the resulting tapes won't be sent to the high score server anymore (where they will be invalidated anyway).
The main difference here would be that you cannot find out which score you
would have reached it you
would have played with regular time limit. But setting the score to "0" once the time reaches "0" (without the player dying, as it would normally be the case) seems to make sense.
This would prevent the need for three (potentially confusing) options, and seems to work reasonable in both cases (with option set to either "on" or "off").
What do you think about it?
Then I would like to comment on this one:
(thinking loudly) i guess that if i'll make the patch, Holger will be way more interested — because then he'll have almost nothing to do. now i need to find some time to make that patch.
I think you are wrong here.
Although providing me with a patch ready to be applied to R'n'D is generally never a wrong thing, the
real work is almost never in writing some new or changed code. The real, hard work (at least for me) is almost always
thinking about it! That is, making sure that no bugs are introduced. Making sure that no existing behavior is broken, or changed in a way that is either incompatible or unexpected for the user (new and existing ones). Making sure that the usability is not affected in a negative way. Keeping things consistent. As the code base is really very old and very complex (at least I think so), it is very easy to break things by a quick change or addition. So thinking about changes is really important, and usually takes far more time that writing the resulting code.
