I think it's worth considering at least? The IaMP caster going with 7500 sounds deliberate and along the lines of being close enough to the common 8000 port, and 10800 is out there enough that it makes sense for that to be the connection. If there were another data point (like if HM did it too, etc) it would be a lot stronger. If you're looking for my thoughts...
Being considered for 10.8 instead of .3 (as Soku was) suggests maybe that it was intended to be released between the so-called 10.5 and 11, meaning that ZUN had both in his pocket when Tasofro began. However, the development train of 10 -> 10.5 ->11 had no actual gaps -- Mountain of Faith released in August 2007, alongside Scarlet Weather Rhapsody's first demo, which itself was released fully in May 2008, alongside
Subterranean Animism's trial, which itself released fully a few months later.
Taking this into consideration, note that by the time of SWR's first demo it was already numbered 10.5. This means that in one scenario for this theory, ZUN would have had to be planning/developing for 10.5 while developing MoF, and then realizing that he wouldn't be able to finish MoF as well as that game before Tasofro opened with SWR, allowing them to take 10.5 and shelving his game for later. Except, this was the golden period where SA would come out the next year, and in the meantime we had ZUN working on SSiB every month, SaBND every month, and CiLR every three months. There's no way he was bothering with such a game at this point, which makes it hard to believe that he might have been planning for it so far in advance and not doing anything with it until way later.
Another flawed idea is that rather than the 10.5 game having to be shelved for later, it was MoF that was pushed back, and it was the other game that was made first: Shoot the Bullet. This would make StB 9.5 instead of 10.5, giving Tasofro the number. But StB was out at the end of 2005 and was originally planned for that Summer alongside BAiJR
and PoFV, which makes it wayyyy too early (and still too tightly packed) to be possible to have MoF also in hand. Besides that, this is when ZUN had to take a break from doujin activities for a while.
Rather, we know that MoF was in the planning phases in
late 2006. It's in this post that ZUN says he's coming back to Touhou after his break, and gives us one of the first examples of him saying that he really doesn't like going back to old work (this being the reason StB didn't get its planned expansion or PoFV getting more patches, even after his break). He more or less says that during his break he eventually got carried away with new ideas that ended up spiraling into a new work; this would become Touhou 10. He then says it would probably be released next Summer, which it was.
So like, we have a pretty solid mental map of ZUN here. I don't see any immediate room for such a game, so if anything it would have really had to be morphed into something way down the line. Even still, you could make the simple argument that Tasofro just wanted to keep the numbering open just in case ZUN wanted to make a new side game, but then ended up not needing to. I suppose that's possible, but all the interesting meaning is lost in that case.
That aside, to support the subject of ZUN never wasting stuff, there's this nugget:
JS: Have you ever started a Touhou game and didn?t finish it? Are there unreleased Touhou?
ZUN: Because I?m making them on my own, obviously I?m not really beholden to other people. So the only time it happens is when I play it and then I go, ?Oh, this isn?t much fun.? So I?ve had games in the series, or I?ve had times, where I?ve gone back and made changes. That happens and it?s a process of improvement. I don?t think I?ve ever cancelled a Touhou game. Every Touhou game that I?ve set out to make has been released or come out.
So this would also support that a theoretical 10.5, if it ever existed, would have later made it as a different game.
oops i made a big history dump