Problem writing custom preseed.cfg

[Richard Owlett <rowlett@cloud85.net>]

I'm attempting to write a custom preseed.cfg file in order to 
disable the installation of any bootloader (target box has 
heavily customized bootloader which I manually reconfigure as 
required).

In the past I've been pointed to template file of an appropriate 
udeb.

I went looking on 
https://packages.debian.org/stable/debian-installer/ .
The only udeb that appeared relevant was nobootloader. Its 
template file was not helpful.

Two questions:
1. In this particular case how do I disable the 
installation/configuration of ANY bootloader?
2. The general case - how do I discover which udeb is relevant?

TIA