From what I know, if the stem ends in -p [p], -t [t], -k [k], then after the -á affixaction, the last sound of the stem becomes voiced (in the case -p [p], -k [k] it becomes [b], [g]; in the case of -t [t] it becomes a flap [ɾ]) and then geminates. What happens when the stem ends in -h [ʔ] (the glottal stop)? Does it also get geminated when followed by -à affix? It's hard to imagine how would a long glottal stop [ʔʔ] be pronounced.
Or is [ʔ] deleted when the -á affixation happens? At least, according to the tone sandhi rules, the syllables having checked tones 4 and 8 which end in -h [ʔ] change their tone to the unchecked tones 1 and 7 respectively (and syllables having unchecked tones can't have a glottal stop at the end).