Provided that you are running ASA software, preferably 8.4 - 9.1, then you can configure the NAT so that it chooses the eggress interface for certain traffic and therefore traffic gets routed using that eggress interfaces default route.
Naturally this is a bit "special" setup but traffic can be directed to different ISPs on a source host/network basis.
And naturally the overall setup of the network defines if this can be done or not.
What you are saying would hold true if the destination ip address is known. Source based routing would still not work on ASA.
I guess the requirement here is that they need to create a source based route since the destination is internet and not some well known address. So, anything coming from inside would go out through the defaukt gateway. The behavior you mentioned to allow ASA to decide egress interface and not doing a route lookup holds true for versions 8.2 and lower as well.