I agree, you can offset, change by 500ft and then divert. But I would add that your SA for this needs to question if doing so becomes a greater threat (ie crossing a load of busy tracks) than descending (or in your hypothetical situation, climbing) out of HLA before diverting.
You’re diverting so maintaining a cruise altitude becomes less of a priority. You may want to jettison (again at 500 feet spacing across tracks? Perhaps not ideal). At FL 285 your TAS will still be pretty decent but GS may also be a consideration.
Essentially this becomes an SA and threat analysis decision, but the regs do allow it