well, a 25 shouldn't be too heavy at chest level if you can hold a 10 behind your head. plates behind your head are MUCH harder than doing them at chest level.
You could try bands if you can hook them on there. i've done bands, plates to chest, plates behind head, dbs, etc. I think i remember doing 35 or 45lb plates to the chest, but couldn't do 25 behind the head.
16 Ways to Maximize your Glute Ham Raise
there's a lot of ideas in that article.