You're right. It is not the optimal solution. Better to get the right scanner and get good results easily.
I did say that, first thing out. I said it was a "tricky" solution that might not work well. It was clear, I believe, that this involved fiddling and I did say that it is best to get a scanner that is up to the job.
I thought I was clear about that but, maybe, I didn't phrase things well enough. I'll try better next time.
However, I don't think it's useless to experiment.
I used to sit in my spare time, tying knots in pieces of rope. People used to make fun when I said that I was practicing my Marlingspike. Then, there came a day when somebody asked me to bridle the loose end of a pipe on a fly line above the stage where I work. About three minutes later, I lashed a piece of wood to the pipe and made a king post truss. Nobody ever laughed again.
Maybe this mirror trick isn't going to save the day but it is experience gained that, even if it isn't directly put to use, can be used in conjunction with other knowledge.
I'm pretty sure, by the way he phrased his original question, that the O.P. knew his scanner was not the right tool for the job. But I am also pretty sure, by the way he wrote, that he is the kind of guy who is willing to seek knowledge for its own sake.
Absolutely! Buy a scanner that can scan film. Spend just $100 and you will have no more fiddling.