neither are "use a /48 from each of the upstreams"
Why not? All of the upstreams is probably overkill, but from three or four big ones?
For optimum routing, and neutrality, at least in the case of the RIPE NCC (and maybe some of the IXes service networks), you want all upstreams (90) to reach their network directly, not going over other peers. So you either take 90 /48s, or take one, and announce that to all the peers/ upstreams (which is mostly the same in these situations).
Note that we are not talking about 90 /48s here, but 90 /64s, one from each of the peers. So, you connect your DNS server with an 802.1Q compliant NIC, two-thirds of these 90 VLANS already exist in the exchange's L2 fabric. Where is your problem?
I can very well imagine a scenario where ISPs are willing to accept more-specifics *in their own (RIR) region*, and filtering out those from other regions. Except for global players (that can't say "this is my region" because they're present everywhere) this would result in near-perfect connectivity to all networks, while not being forced to take all the crap.
Gert, What you describe is a geographically based prefix filtering system. This does _not_ belong into PA space. It belongs to geographically aggregatable space, which like its name implies will be not only filtered based on geographical criteria but also aggregated. We have a project in ipv6mh called "geo for now" which is similar to what you describe and that we think is the short-term answer you and others are seeking. Aggregation of PA can _not_ be broke. Please allocate the resources of your brain into getting the solution out instead of futile attempts. Michel.