The address and data bus pins can be assigned in a manner compatible with the SRAM board. That still leaves 12 or so CPU-specific control signals to place. The CPU also needs a clock signal, so it makes sense to assign the control signals on the header for I/O group C. This in turn suggests that groups A and B are most suited for the CPU address and data busses.
The V Automation website sells IP for a Z80-compatible core that they say requires less than 8000 gates. If true this would occupy 4% of the XC2S200. I find it a bit hard to believe since the ZX81 glue logic alone occupies 3% of the XC2S200.
The Free-IP website have packed their Free-6502 v0.5 core into a Xilinx Virtex chip (XCV100E-8), taking 557 slices and ran at 39 MHz. VAutomation's Z80 core takes about twice the number of gates as their 6502, so we could guess that a Z80 would take 1114 slices. The XC2S200 has over 2,352 slices and Bodo/Kai's VHDL consumes 91 of these. At a guess, a Z80 core thus might occupy 47%.
I have heard of Z80 VHDL in the public domain but the author says that although it compiles it may not be synthesizable. Which means that although it seems entirely logically possible it may not be practically possible. I get the feeling that it may be relatively easy to get most of a Z80 designed, the rest of it is increasingly awkward. Zilog didn't seem to produce streamlined elegant designs, they just bolted on features that they thought might be a good idea. Rabbit Semiconductor noticed this and threw out the less useful parts to produce a core that was more efficient (computing power per silicon area) and still reasonably Z80 compatible. Legacy Z80 code may well use such infrequent or undocumented opcodes, and it is going to be extra work to ensure 100% compatibility.
It doesn't seem worth the effort for a CPU that cost about 75p in bulk. Having said that, I hear Zilog are filing for bankruptcy so unless someone think it is worth producing them then eventually a VHDL version may be the only option. Have you tried to buy an 1802 recently?