How to initial onchip_memory in modelsim post-simulation?

The ATOM simulation models don't read the HEX or MIF directly. Rather, as you've discovered, they load their contents from the mem_init<num> generics. If you update your HEX or MIF, run processing -> update memory initialization file, then generate your VHO. The mem_init<num> generics will be updated to reflect your changes. Be careful about trying to correlate the generic values to the original file. The mapping isn't documented in the Quartus II software and depends on the way your logical RAM was organized into individual RAM blocks.