Checked the version of the Dosbox pure libretto core in Retroarch. It is using version 0.9.9 from december 2023. Have to wait for a stable 1.0 release and then probably a libretto core will be compiled until then it will not be possible to create an internal drive letter without the use of Dosboxpurex.conf.
Have enabled Online Updater and updating experimental cores in Retroarch. Updated 96 cores to the latest version but alas DosBox Pure is still version 0.9.9. No experimental DosBox libretto preview cores. :-(