Supporting the select helpers in transmute would make the syntax more confusing, as a given argument could then refer to a single column or a collection of columns.
@mungojam, am I right that the majority of the need for select helpers (specifically everything()) in transmute would go away for you if you could choose to insert the new columns from mutate on the "left" side of the data frame? That seems like a smaller ask, in that inserting columns at the end/right doesn't seem inherent to the idea of creating a new column.
The implementation would be a little tricky, in that adding an argument to mutate could potentially break existing code that had a column named with the same name as the new argument. The alternative would be a mutate_left or similar.
Actually, you could arguably just define a mutate_left function yourself and stick it in a utils.R file/package, if it's something you regularly do.