Admittedly I'm yet to try my OTGW with the boiler/thermostat (I've been told to wait until spring before tinkering), but all appears to work otherwise. So thought I'd write up the process here if it's useful.
1) Switch the FT232 to 5v mode (on my red ones there's a jumper near the serial pins)
2) Set up the 16F88 and a FT232 adapter onto a breadboard, making the following connections with jumper wires:
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FTDI -> 16F88
---------------
VCC -> VDD/14
GND -> VSS/5
TxD -> !MCLR/4
DTR -> PGC/12
RxD -> PGD/13
CTS -> PGM/9
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FTDI -> 16F88
---------------
VCC -> !MCLR/4
GND -> PGM/9
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# 0 TXD / 1 RXD / 2 RTS / 3 CTS
# 4 DTR / 5 DSR
# 6 DCD / 7 RI
DEVICE=FTDI
IFACE=0
BITRULES=0x1000
SLEEP=0
VPP=0
PGC=4
PGD=1
PGM=3
What this does is change the PIC configuration the HEX file sets to Low-voltage-programming. Because we're using LVP the PIC won't let us set it to HVP, and the write will fail otherwise.
6) Write the new .hex file:
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./src/p14 id
./src/p14 program gateway.hex
./src/p14 verify gateway.hex
Because we used LVP, we need to connect PGM to GND before the PIC will boot. This means we lose out on a LED, but I'm putting this in a cupboard anyway.