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I use XOOPS as part of my company's hosted portal product - OBS Portal Server. It is Xoops, IBM Domino, Icewarp Webmail, OBS Webdesign, IBM Portal Server, IBM WebSphere Commerce Server, and some other items all integrated. It is a hosted only option. Because of the GPL and my use of close sourced applications, I can not release it.
I did the exact same thing as you are talking about. I currently have a custom XOOPS image with 40+ hand picked modules. I have included some custom hacks, custom themes, new module icon set, enhanced admin interface, and more. I allows me to have a very consistent install. Very little of these improvements are mine alone. Most are pulled together from all of the different resources I have found here.
I think that what you are planning is a great idea, it has served me very well with zero problems.