I could indeed add extra filters in the "open table" window. This has been planned for a long time. Since the inn was introduced really. But it has had less priority than other things unfortunately.
Perhaps the priority level should go up? Not trying to tell you what you should or should not do, and I certainly don't know what else is on your To Do list (which I'm sure is quite full). I'm just judging by (1) the posts that are here, most of which seem negative towards being matched with "whoever," and (2) just knowing that lots of people on other gaming sites feel the same way. The difference with the other sites is that they don't "match" you to anyone. People just accept invites as they choose, and while filtering exists, there's also room for notes (for requests).
Yucata added a filter that allows you to choose a +/- level relative to your own rank, e.g. +/- 200 of me. BGA has rank range names, like average, good, expert, and you can filter on that. I think both methods need tweaked a bit.
I think I like Yucata's concept better. Having a filter like 1101-1200, 1201-1299, etc. (which is what BGA's method amounts to) means if you're 1199 that could be a little screwy, so the +/- a range relative to yourself works better. But they would have to be smaller ranges than Yucata, like +/- 50, 75, 100, etc.
I don't know how difficult that filter would be, or how difficult it would be to make it "open," like a player entering whatever they want, such as 1225 - 1310. The nice thing about filter ranges, though, is that different people will have different ideas as to what constitute an "appropriate opponent," and it can be totally left open for personal invites.
If it's easier to let the system decide what range is appropriate for you, two possibilities to allow for personal invites is either just an option to turn it off for a game (or on, depending on default), or have it automatically turn off for personal invites.
Just as a funny note, I'm not sure what time these people are playing, but when I'm here there's never more than about 6 tables open and I've only once gotten matched with a human before the timer ran out and let me play with a bot.