TEI Guidelines recommend using <gap reason="deleted"/> in case of unreadable deletions (cf. a note in the description of <del> http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-del.html\),
but <gap> is not allowed within <subst>, which the tag designed for grouping deletions and additions (its content is model.pPart.transcriptional).
I wonder whether adding model.global.edit to model.pPart.transcriptional would not be the best solution. It looks logical and might solve other potential problems.