Apportioning GST in the account register

[Steven Patrick]

Good day,

I have a client who wants to use Gnucash as a cashbook and be able to 
apportion GST on certain entries in the cheque account register.  The idea 
is to have some options for "Taxable?" and "Tax Included?" like the Invoice 
Entry window has and then automatically apportion GST to the appropriate 
account if the transaction is taxable.

I have spent numerous hours digging around the Gnucash source, only to 
realise that it's a lot more complex than I thought.  So, my questions are 
as follows:

1. Can anyone give me a clue as to which source files I should be looking at?

2. If I can't come right with this myself, would anyone be interested in 
paid development work to implement this?  The results of the work would be 
made available under GPL again.

Kind regards,
Steve