I know of a diploma thesis  project that deals exactly with this issue. It contains a working CDI extension (called Classic) which acts as a bridge for Seam 2.
"Classic [..] is an alternative implementation of the Seam core. More precisely, it is a portable CDI extension, which enables a Seam application to run in a Java EE environment without Seam libraries. [..] Where possible, [seam-specific] metadata is replaced by matching Java EE substitutes. As a result, parts of the required services are provided natively by an application server. Secondly, the Classic extension provides a compatibility layer, which emulates the remaining services at runtime."
I haven't tested it, but it might be worth looking at.