Hello Rob,
First: it seem to me this post went into the wrong group - this group is for SAP Business Suite on ASE.
Your post definitely refers to an non Business Suite application. I suggest to use the all purpose group for SAP Adaptive Server Enterprise in the future. (SAP Adaptive Server Enterprise (SAP ASE) for Custom Applications )
Anyway, this sounds it could be a tricky task to accomplish.
Recovery Fault Isolation (RFI) could perhaps be an option. There have been limitations in RFI , which have been removed only very recently - so you might have to install the very latest 15.7 (15.7 SP136 HF1) , then set up RFI and then attempt to perform the LOAD of the 12.5 DUMP , using TF12700 to boot ASE . I have no idea whether that will work.
One last warning regarding by pass recovery: The database is definitely corrupted (transactionally inconsistent) after such a treatment.Whatever you get out of such a database will have to checked by someone who knows the application logic and can (try to) manually restore transactional consistence. (as far as possible). Such an approach is absolutely impracticable for a database of an SAP system !
So bypass recover is not an option for SAP databases.
Good Luck.
Tilman