Glad you have some workable code to play with.
In my test, ACAD2000, the dxf 7 did not care if the case did not match.
This get nothing?
(setq ss (ssget "X" '((0 . "MTEXT")(7 . "helv14"))))
BTW you can also do you code this way, just FYI.
(setq ss1
(ssget "_x"
(list
'(0 . "MTEXT")
'(8 . "TITLE-BLOCK-TEXT")
(cons 410 (getvar "ctab"))
'(-4 . "<OR")
'(7 . "helv-meds-19")
'(7 . "helv14")
'(-4 . "OR>")
)
))