I don't consider it technically unjustifiable. I consider it grammatically incorrect.
Reflexive pronouns sometimes behave in strange ways in prepositional phrases. Have you ever had a learner ask whether to say, e.g., "She took her cat with her" or *"[strike]
She took her cat with herself[/strike]"? The second sentence is clearly ungrammatical, at least in a normal context (without exaggerated emphasis on "herself"), yet the customary advice about using a reflexive pronoun when the pronominal object of a verb or preposition is co-referent with the subject of the clause would lead us, inappropriately, to use the second sentence instead of the first. At least one dissertation has been written about conundrums in that department.
In the sentence of mine that you object to ("I'm curious to know whether other native speakers of English besides myself are comfortable with judging the (b) sentences grammatical"), the reflexive pronoun ("myself") may not be co-referent with the subject of its most local clause (the "whether"-clause, the subject of which is "other native speakers"), but it is co-referent with the subject of the sentence as a whole ("I"). Thinking about it, I believe that makes a difference to my perception of it as
not incorrect, at least in my dialect of English (native Californian). We could test whether our judgements are similar here.
We agree that (3a) and (3b) are correct. Sentence (4a) was my sentence from the OP. We know that I'm OK with it and you are not. I'm equally OK with (4b), in which the reflexive pronoun has been changed and the subject of the highest clause has been changed to "agree" with the reflexive: "I"-"myself"; "He"-"himself." What I find interesting is that I am
not OK with (5a) and (5b), in which the reflexive pronoun "agrees" neither with the subject of the most local clause (the "whether"-clause) nor with that of the highest clause: "I"-"himself"; "He"-"myself." Perhaps we could agree that (5a) and (5b) are worse than (4a) and (4b). Do we?
(3a) I'm curious to know whether other native speakers besides me are comfortable with judging the (b) sentences grammatical.
(3b) I'm curious to know whether other native speakers besides him are comfortable with judging the (b) sentences grammatical.
(4a) I'm curious to know whether other native speakers besides myself are comfortable with judging the (b) sentences grammatical.
(4b) He's curious to know whether other native speakers besides himself are comfortable with judging the (b) sentences grammatical.
(5a) I'm curious to know whether other native speakers besides himself are comfortable with judging the (b) sentences grammatical.
(5b) He's curious to know whether other native speakers besides myself are comfortable with judging the (b) sentences grammatical.