The guys at Developer Express support say that having a two-way association in your object model is fundamental to XPO multiple associations and, as my request was only to simplify code, the feature is now not to be implemented. The bottom line from them was:
"To sum it up, we think that the current design of one-to-many associations in XPO is reliable and transparent. We are not going to change it, unless we have strong arguments for doing so."Well, although it's a shame that it's not going to be implemented, you can't argue than that!
You can find the request thread here (but may need to register). If you still want to keep your XPO code clean, you can use some IL enhancement with PostSharp - details here.