We also need a simple invoice creation and accounting program for our business, and it seems to me that the program that we currently use for this (Instant Invoice, a Windows app that I run using Wine) is very similar to HomeBank. Perhaps you could be convinced to either add some invoicing features to HomeBank or create a separate version for this purpose? I have been looking for Linux software to replace this Windows app for months and I haven't found anything like it, so I expect that a lot of other folks would be very happy if you did.
Basically the software I'm looking for is almost identical to HomeBank except that it's focused on (or includes) a list of invoices instead of bank transactions. Instant Invoice allows you to create simple invoices (it keeps a list of customer accounts and can also store product info, although I personally enter in descriptions and amounts every time), displays them in list and report form, and also allows you to enter payments for each. Take a look at it and I think that you'll see that the structure of the program is very similar to what you've already done in HomeBank, so it might be just as easy as creating a basic invoice module that would allow users to create transactions as invoices and viola you've got a simple invoice program as well as an accounting one.
Please consider adding this to HomeBank. I think that it might expand your user base to a lot of people with small businesses or who do contract work. I know that I would appreciate it very much!