... is indeed possible: simply change the address decoder. For example, by changing the GAL equation for the decoder from 

RAM_SELECT = !A11 & !A12 & !A13 & !A14 & !A15 ;

to 

RAM_SELECT = !A11 & !A12 & !A13 & !A14 & A15 ;

the emulated 2 KBs of RAM will appear in the 0x8000 - 0x87ff address range instead of the default range from 0x0000 - 0x07ff. 

This hence gives you 2.5 KBs, adding 512 bytes from the existing SRAM chips to PicoRAM's 2 KBs. Note that the 2 2114 chips are on board here, and don't conflict with PicoRAM. It is probably even possible to pull the ROM chip (0xfc00 - 0xffff) and hence change the monitor program if needed (i.e., by putting a custom "emulated ROM" in there that can conveniently be updated and developed without having to burn (E)EPROMs all the time):