There is no notation for the tab character in FORMAT
.
There are several choices, but none is really really good.
use
#\tab
(or a variable set to the character) as the argument, as you mention, is okay for meembed a literal tab character in the string. This may break with some editor settings, where the editor replaces tabs with spaces. It's also not directly visible.
use a function in a format string, which writes a tab character
use a reader macro to introduce extended string syntax. Probably not bad. Maybe there exists even one. There was a post on comp.lang.lisp with an example.