Yes, got the pcore working! At the end of the last read before the z-interpolation state, the Mealy state machine assignments keep the rd_req signal high, which is also the mst_go stimulus.
When axi_done is asserted, it takes one cycle for the rd_req to go low, but this is enough to start ANOTHER rd_req. When the mst_sm reaches the state where it wants to latch the rd/wr req value, none are high, which is why it stalled.
Tested only for length 256 case. Will test more.
Merge branch 'master' of https://github.com/molohov/gpu2