I haven't tested it but you should be able to parallel the outputs. Normally balancing resistors are required in order to connect transistors in parallel but in this case the transistors are on the same piece of silicon and should be pretty well matched. You should still observe the maximum total emitter rating though.
Um, just to bring this thread to a more complete close, I can now confirm - as hoped and expected - that the ULN2003AN is a direct functional swap-out (for the purposes of my circuit) for the ULN2803A - 'cept it is slightly smaller.
**broken link removed**
I will be using these - instead of even two transistors, I'd say - from now on.
I got a tube of 25 of them for £6, so you can't complain at 24p each.
eBay... guy / shop had an auction for 50, but he was OK with doing me 25. The price I paid was including postage and from order to arriving was 1 day. One day I will order something from Rapid, but their delivery charge often means I go elsewhere, rather than being patient and allowing an order to build up to +£30.