I was able to find some of this information originally on the internet, but 
it was incomplete as it only suggested wiring 30-32 together in order to use 
an EPROM in place of the NEC Masked ROM in an 8300.  

Thanks to Kyle Jenne for filling in the rest of the pieces on the other pins 
that needed to be jumpered together.

I have personally verified that this solution works in one of my NEC PC-8300s,
so this is definitely a solution for creating an NEC PC-8300 replacement system
ROM using either of two EPROMS mentioned below.

Compatible EPROMS: NEC 27C1000 & HN27C301G. Those are the ONLY chips you can 
use. They are 32 pin 1M EPROMs but are pin compatible with 28 pin 1M uPD231000 
mask ROM.  

1. Burn your 128K ROM image into the suitable chip.

2. Solder a jumper wire connecting Pins 1&2 (Vpp and OE) together and to 
   Pin 16 (GND).

3. Solder the not connected Pins 31&32 (/PGM & Vcc) together and to pin 30 (NC)
   to pick up VCC from the socket below (ie 30&31&32 together).

4. Insert correct orientation in ROM 0 socket, leaving Pins 1&2 and 31&32 of
   the EPROM hanging out in mid air near the bottom edge of the computer.

MASKED ROM                            EPROM 
(1M) 128k x 8 mask rom (TC531000)     (1M) 128k x 8 EPROM (HN27C301 or NEC 27C1000) 
(23C1000)                                                 
                                    / Vpp    +-v-+ 32  Vcc   \       
                                   |\ OE   2 |   | 31  PGM/ -|  <-- Wire pins 32, 31 and 30 together, getting Vcc from pin 28 of socket
A15  1 +-v-+ 28  VCC               |  A15  3 |   | 30  nc    /       
A12  2 |   | 27  A14               |  A12  4 |   | 29  A14            
A7   3 |   | 26  A13               |  A7   5 |   | 28  A13            
A6   4 |   | 25  A8                |  A6   6 |   | 27  A8             
A5   5 |   | 24  A9                |  A5   7 |   | 26  A9             
A4   6 |   | 23  A11               |  A4   8 |   | 25  A11            
A3   7 |   | 22  A16               |  A3   9 |   | 24  A16            
A2   8 |   | 21  A10               |  A2  10 |   | 23  A10            
A1   9 |   | 20  CE/               |  A1  11 |   | 22  CE/            
A0  10 |   | 19  D7                |  A0  12 |   | 21  D7             
D0  11 |   | 18  D6                |  D0  13 |   | 20  D6             
D1  12 |   | 17  D5                |  D1  14 |   | 19  D5             
D2  13 |   | 16  D4                |  D2  15 |   | 18  D4             
GND 14 +---+ 15  D3                 \ GND 16 +---+ 17  D3