Thank you for the question. Full disclosure, this is probably more of a Braille Formats question than a UEB Math/Science question.
Nonetheless, I'll go ahead and say that I like your proposed solution. Instead of repeating the same tn at the beginning of every blank-riddled table, a tn on the Transcriber's Notes Page saying something like, "In all tables, a series of guide dots across the width of a column indicates that an answer is to be inserted."
Before you implement that solution, we must ask: Are there any tables with a blank that's not meant to be filled with answers?