Where are you getting the extra 25lbs?
I've got a setup at home too. I previously had a set of standard plates before the olympic set. I use those to add weight to my deadlifts on the olympic bar. I tie these on using a chain. I got the idea from an old photo of Bob Peoples deadlifting at a competition where they had tied plates to the bar for his lift.
Try something similar. Buckets of water, milk containers, etc tied to the bar. The back pack idea isn't really the same lift.
Or you could just do more reps or buy more weight......