Conversion Rate Optimization can surely be done on most vendors. I've done it on Dealer.com sites and they are one of the most limited as far as any kind of advanced implementations.
It basically consists of using custom filters and advanced segments in Google Analytics to capture everything about how each visitor is interacting with your site when they either open a contact form, then close it or open it and fill it out. This also includes every visitor who clicks on any call-to-action on the site and then either close it off or submit it. You combine this data with bounce rates, time on page and a few other things, which ultimately give you a pretty crystal clear picture as to areas where you need to re-design, re-format or change the placement of your CTA's and also any problems with your contact form such as unnecessary fields, too many *required* fields, poor wording and ordering of fields.
You then basically change little things here and there and see how the data reacts. So any site that will allow Google Analytics and basic HTML modifications is CRO-able.
However, you are entirely correct in saying that SEO is very limited with a lot of vendors. Anything that requires changing the code is a no-no. Big things I've ran into were with the site architecture, page extensions and setting up things like canonical tags.
Dealer.com has those pages where it might be something like
http://www.website.com/new-inventory/index.htm ... and no matter what you put after index.htm it still works, such as "
http://www.website.com/new-inventory/index.htm?hello" or
"
http://www.website.com/new-inventory/index.htm?howyadoin" etc. they are all leading to the same page. The problem is after a while, Google indexes several of these URLs and to Google, they are all separate pages with duplicate content and this is BAD. The canonical tag tells Google, "hey,
http://www.website.com/new-inventory/index.htm is the main page, so don't count those other ones as individual pages."
A giant reason I started my business is because if dealers don't start addressing SEO seriously and moving to vendors that allow them the flexibility to do so - they will seriously find themselves being DE-INDEXED from Google, straight up. I've seen it happen from the Penguin update and that is nothing compared to the lengths they are going to go in the future to stop people from gaming the system. My partner built websites for one of the biggest tech companies in the auto-biz for 10 years and my plan is to eventually start getting dealers switched over so we can do their SEO, their websites and everything all in one spot with one clear strategy and it will be highly effective - I just hope they start opening their minds a bit and allow capable people with good intentions to help them.