Can the HPS EMAC RGMII on the cyclone V be connected to an external MAC interface?

My apologies -- I was actually confused earlier about how this example outputted it's signals (for some reason I thought it would output them as A/B/C/D differential pairs, but it clearly does not) -- I think you're right that this is applicable to what I'm doing and I'm currently trying to implement it.

I think my issue might have to do with timing, but with this HPS-to-FPGA setup I can play with delays on the rx and/or tx clock lines and maybe get it to work.

It is a custom board -- I have the single-ended RGMII I/Os routed to a KSZ9897R switch IC, specifically port 7 of the switch, which is a MAC interface port.

Thanks for the link, although I've seen and reviewed this example before. The FPGA IP's that are used ("HPS EMAC Interface Splitter Core" which feeds into the "GMII to RGMII Adapter Core" IP) actually only output differential PHY RGMII signals, so it's not something I can use in my design's MAC to MAC configuration.