How to initial onchip_memory in modelsim post-simulation?

I'm still a bit perplexed that you want to debug the post-fitting netlists in Modelsim. It's like debugging a C++ after its been compiled to heavily optimized machine code. I'm all for simulating a post-fitting netlists, but I generally dread tracing into the netlist to isolate a testbench failure. You should verify your RTL in Modelsim, where you can work out any functional kinks in your design. The post-fitting simulation should be a sanity check. If it fails, any number of issues could be at fault. For example, you may need to add/modify timing constraints, or you may need to review the Quartus II warning messages for any design issues. Look especially close at any critical warning messages. I'd explore all these avenues before I ever start probing internal signals in the post-fitting simulation. Sometimes it's almost easier to route those signals to top-level debug pins rather than make sense of the compiler-generated names.

Regarding your custom IP: I suggest you verify it separately, then integrate it into your full system. If something doesn't work, then follow the steps out lined about - check your timing constraints, look for suspicious warnings.