Hi rambal,
there is a sample app that uses MXML config here: http://www.springactionscript.org/samples.html (see "Movie-app with MXML based application context")
Basically with MXML you have the option to skip the creation of object definitions and you can declare your instances directly, as you would normally do in MXML. There are some limitations though, mainly with prototype scoped objects, but in your example you should be fine.
Code:
<mx:ChannelSet id="remotingChannelSet">
<mx:channels>
<mx:AMFChannel id="amfChannel" url="http://localhost:8090/dmi/messagebroker/amf"/>
</mx:channels>
</mx:ChannelSet>
regards,
Christophe