I think your biggest problem is that subinterfaces on ethernet ports are (I'm fairly certain) reserved for use in VLAN scenarios only. If you try to create that subinterface (assuming you could come up with some sort of policy routing scheme to meet your needs), I suspect you'd get an error message of some sort. Actually VLANs would be your ideal solution...but...that 2500 won't support them (again, I'm *fairly* certain about that but I hate stating negatives in the affirmative).
If I understand your requirement correctly, I think you need a hardware upgrade. I guess some sort of secondary address scheme vs. a subinterface scheme might be doable. But they're messy and have a lot of restrictions so I don't deal with them, so I can't be of much help in that regard. Anyone else think of anything?