I think that it's not possible natively. But there could be two ways to achieve the same thing, but it's questionable if these ways match your needs:
- On the ASASM there are private VLANs supported which could give you the needed functionality. Ok, the pricetag is a "little" higher than a 5500-X.
- If you don't mind adding a lot of complexity and some additional licensing cost, then you could achieve a similar thing through virtual contexts. The customers each get a transparent context to an external router or a cascaded routed-context.
If you find any other solution, please post them here. I'm also searching for a suitable solution for a hosting-environment where we don't want to waste that much IP-addresses.