I finally found the post from aifarchive that I was thinking of (dated June 15):
"Would I like to see an SD3 Expansion? Sort of... In fact one is (sort of) planned.
"I have actually got a v1.1 which fixes the various bugs people have mentioned. I haven't released it yet because a) I've been busy with the minicomp, and b) There didn't seem to be as many bugs reported as I thought there had been and I was wondering if I had missed some and needed to go back and check through all the emails and... see part a). I'll release it soon, though... When I get round to testing it. (Unless anyone wants to volunteer?)
"As for an expansion... I have a vague notion in my head to release SD3 Gold version. This would have some more content in, though I am not sure how much. ABomire mentioned he had written some more daydreams... a few of the suggestions people have made on the boards might be possible (an alternative path for Alison being the main one, and maybe a player-only Molly ending). My concern with extra content would be the effect it might have on the game structure, dynamic and theme. Oh, and the amount of work it would take - specifically how much of the existing game would have to be rewritten or added to. Take, for example, if I decided to make Yuki a more major character and have her at the party (I won't, by the way!) - this would require major reworkings of just about every interaction the player has with Yuki, and with other girls if they find out about it, and the code for the party itself would balloon and Yuki would need answers to all the questions, interactions with all the characters, the other girls' reactions to Yuki... I could have written a medium-szied game with the amount of code I would need to write just to do this.
"In short, any expansion I write would need to slot into the existing game without changing too much of the rest of the game. A Molly only ending would be very doable - nothing would change until near the end of the game, and then it would just branch to a Molly scene similar to how it does for the Becky only ending (I would still have to consider how/if it would fit in to the game, though). An alternative path for Alison would be more work, due to the knockon effects this would have on the other characters (although this is probably the expansion I would most like to do). But I'll see.
"Either way, this will *not* be done for a while. Probably a long while. I'm back at work on GoP3 now, and want to get that done before I go back to SD3. And maybe by that time I won't want to do it anymore!
"As for other people releasing an expansion... I don't think so. Firstly, I think it would be very very difficult to do. Even if I gave someone my code they still probably wouldn't understand it (About 50 files, all uncommented, with stuff scattered everywhere). And if they did, then changes like the ones mentioned in a previous mail would be very very difficult to implement. And even if they were, I think it would detract from the game. SD3 probably has too many diversions from the main plot as it is.
"I hope that is a good enough answer.
"But, thank you for the interest though! I'm flattered that people would even be discussing this

"Cheers!
"GoblinBoy
"PS - SD4? No plans for that I'm afraid."
So there you go, version 1.1 will be bugfixes only (not sure why it hasn't been released yet, hopefully it means that Goblinboy is too busy with other projects). An expanded version is a remote possibility, but don't hold your breath.
For what it's worth, I happen to agree that there aren't too many places where you could reasonably extend SD3. Sticking in new sex scenes just for the sake of them would distort the story as well as being a hideous amount of work.