If you have a choice about it, I would recommend OpenACS 5.0 as your target platform instead of 4.6. It's a huge leap forward, and better to target that than 4.6 and upgrade later IMO.
I think as you hang around here you'll find that all of us are busy satisfying our customers as well, and our focus is similar. Most of our development is client driven.
There are some issues to hash out here, but don't disappear until February! If you're planning on porting to OpenACS, you'll probably want some advice along the way while you're getting up to speed on OpenACS.
I don't think we have any problem with proprietary code on top of OpenACS, but generally you're not going to interest very many other people in working with your code unless you release your code under some sort of open source license. And GPL is the default here, so if you're wanting to release it under something else, that's fine, but it'll be up to you to convince developers to jump aboard. If it's in our self-interest, we will, and we'll be happy that you're contributing your code for all of our self-interests (including your own!). It's a win-win situation in my opinion.
Glad to have you here, and I'm looking forward to seeing P/O on OpenACS if you choose that route.