Well, I’m in the choir in my church, and the whole collection thing is a pain in the neck. It was at my last parish, too. I don’t have any family members in the congregation, and since you’re supposed to write the amount of the contribution on the outside of the envelope, I don’t particularly want to give it to someone else. I would like to be able to contribute like everyone else, with the same privacy, and the same ability to give according to my pay schedule (which is not monthly) and not have to make special arrangements. So generally I have to go around after Mass and find an usher. It doesn’t seem that big a deal to anticipate the collection by a couple of minutes, but if it’s against the rubrics (which I don’t know either way) it should stop. If it isn’t against the rubrics, what’s wrong with it?
But to answer your question, why
would you be looking at the choir during (I guess) the General Intercessions? Or at most any other time, either? It’s not a concert. The choir is simply there to help the congregation worship God more deeply and participate in the sung parts of the Mass more confidently. We stand when we sing because it is better for breath control and it helps us project over the church, not so people will look at us. (The exception would be if the choir (rather than a cantor) is doing the responsorial psalm, which we aren’t in my parish. Then it would make sense to be looking at the choir.)
Anyway, I suppose there really are people in the congregation who are watching the basket go around and thinking about who is putting in an envelope and who isn’t, and why it might be that so-and-so did, and such-and such didn’t. If so then they’re probably doing it to the rest of the congregation (the parts they can see) not just the choir. I’m not sure that they should dictate what happens to the choir’s envelopes.
And do the other members of the choir care whether nosy people are speculating on their giving habits? If so, it would seem to be their job to raise an objection. I understand that you are trying to be kind to the choir members that give other ways, but I’m not sure it’s necessary to worry about it.
SMHW, that’s a good idea about having someone pass around a basket before Mass. I’ll probably suggest it at my parish as an alternative to “The Hunt of the Elusive Usher.”

During the Homily, though, I would imagine that the OP would have the same problem as with them doing it immediately before the offertory.
–Jen