USGA/R&A Decisions 20/1 & 20/2 talk about this.

Relief or dropping is covered in several rules but only specifies whether the distance is one or two club-lengths. Clublength is not defined - it is presumably any legal club and could be a long putter or whatever you decide upon.

Nothing in the section on etiquette about not taking advantage of a long putter.

Nothing about clublength in definitions