The use of a BGP Route Reflector at an L2 IXP does not make it a L3 IXP. IMHO - L3 IXPs do not work any more. The "IXs" that use them are really 'international transit services' using the "IX" term to sell their service. The IXPs with lots of 2501s have scaled because of BGP Route Reflectors. The RR is a cheap low maintenance, low learning curve alternative to a Router Server. We know that cause it has proven it self in action for the past four years. Over time, the IXP can transition from a RR to a RS - like some large IXPs are doing now.
BGP Route Reflector IXPs need a AS number. I'll send you a URL with a whitepaper. The BGP Route Reflector IXPs have proved to offer a low entry cost for ISPs (for those places that do not have the deep pockets to get big routers).
except that big routers are not needed for small-isp exchanges. remember, an isp participating in such an exchange has only to add the prefixes of their local peers to their routing, typically a dozen or so. there are very successful layer-two exchanges where the peers use what we think of as cpe routers, e.g. cisco 2501s. and what's nice is that this is on the right path to exchange growth.
l3 exchange ponints are a labor suck and are fragile.