There is no 'code' in the character generator - but a data description defining the individual characters you need. This is fully described in the data sheet (see
http://bytecollector.com/archive/mark_8/My_Mark-8_Info/DigitalGroupPacket/M6570.pdf) or the VTI manual (see
http://www.polymorphic-computers.com/manuals/Poly_VTI.pdf) for various examples.
The Motorola character generator has a few 'silly' voltages on it (other than the sensible +5V that modern EPROMS require - well, modernish).
You would wire the four RSx lines to A0 through A3 of an EPROM and 7 character address lines to A4 through A10 of an EPROM (requiring a 1KBYTE EPROM at most). If you used a larger EPROM - you could wire the higher address lines of the EPROM to stake pins and select different 'character sets' on the fly.
The output from the EPROM will be 8 bits - but you would only use D0 through D6 (7 bits) to feed the '157s.
Each character internally in the EPROM would be formed from bytes - corresponding to row select codes of 0 through 15. IC16 of the VTI is reset when the counter reaches 15 - meaning that only row select codes 0 through 14 are really used. The 16'th byte of each character stored in the EPROM is therefore unused.
You need to watch the 'shifted' characters. In the original Motorola character generator - bit 7 was used to indicate a 'shift character' (i.e. the normal 7x9 character dots would be 'shifted' in the character cell). This data bit was never brought out of the character generator - as it was used internally. In the EPROM implementation - you can quite happily ignore this feature - as long as you remember to code the shifted character on the correct 'rows' of the EPROM. The shifted characters are indicated in the data sheet by a small black triangle in the upper-left of the character cell.
I have done this for a different VDU circuit (a Sinclair MK14 VDU to be precise) and it worked well. It doesn't take too long to enter the character matrix in HEX - and you can do this with an assembler using the Define Byte directive and you can maintain and change the character generator at will.
The data sheet states that the access time is typically 350 ns / maximum 500 ns so any 250 ns or faster EPROM should suffice.
Quite willing to help out further if my description above is unreadable!
Dave