I decided to run this down after working on https://discuss.pynq.io/t/how-to-allocate-pl-ddr4-on-rfsoc4x2-in-pynq-3-0-1/5586/8 given that they’re notionally related.
This patch Add hook for EmbeddedDevice XRT devices to allow moving the PSDDR memory region by ld-cd · Pull Request #1424 · Xilinx/PYNQ · GitHub allows you to put the default allocator in a reserved memory region. Alternatively you can add an .xclbin file alongside your bitstream which has both the PSDDR region at 0 and another region pointing to your reserved region and whenever you want to allocate something in your reserved region specify it manually as a target.