I’ve just read through this thread as I’m in a similar position. In my case my client has one product which they wish to quote on the site ‘inc VAT’ as it’s sold to individuals but there are also other products on the site which are sold to businesses so need to be quoted exc VAT but still have VAT added at checkout.
I’ve set up VAT (For the UK only) in PayPal (15%) and have then entered prices in the simple commerce module minus VAT. I’m planning to then put together a little plug-in which will accept a single parameter (price) and add the VAT for output into a page template.
@Chris: The Paypal plug-in you have written looks great, whilst I’ve not had a chance to try it, I can see it being very useful! You should also perhaps consider submitting your amendments to the simple commerce module to the EE team to see whether they could perhaps be included in the next release?