>> There are other uses of "reftex-" in style/, so I guess something
>> similar should be done for all of them?
>
> Ideally, yes. (But it seems tedious... Does anyone see possibility to
> tame RefTeX to live along with non-file buffer?)
>

Making RefTeX work well with non-file buffers always seemed to me like a
major undertaking.  The commands

;; reftex-add-index-macros
;; reftex-add-section-levels
;; reftex-add-label-environments
;; reftex-scanning-info-available-p
;; reftex-access-parse-file
;; reftex-access-scan-info
;; reftex-ref-style-list
;; reftex-ref-style-toggle
;; reftex-set-cite-format

each call reftex-tie-multifile-symbols, which in turn calls
reftex-TeX-master-file, which fails (with a hard landing) in a non-file
buffer for which TeX-master has not been set to a tex file.

The attached patch wraps the calls from style files to such functions
with a check for buffer-true-filename (although I would also prefer a
less tedious way to address the underlying issue).