It seems to me that what goes inside of an interpolation is an expression, and if you want to filter something, that should be part of the expression syntax. There are already clear ways to spell that in standard expressions, e.g. function calls or method calls (in some cases).
I don’t like the idea of taking an existing operator and changing its meaning completely. (There’s a reason the existing extra notation uses characters that are not operators.)
If you want a filter operator it should be a proposal for an additional expression operator, not just in the context of t-strings.