Version 2.5.0 and some vague roadmap
The latest version of External Import (2.5.0) contains several bug fixes related to MM relations. A few bugs happened when External Import handled the MM relations itself, which happens in some specific cases (otherwise the work is delegated to the TYPO3 Core Engine). In particular properties like "MM_insert_fields" and "MM_match_fields" were not considered.
This version also contains a new hook which makes it possible to manipulate connector parameters on the fly, before the data is actually imported. Please refer to the manual for more details: http://docs.typo3.org/typo3cms/extensions/external_import/Developer/Hooks/Index.html (first hook in the list).
There will most certainly be one or more bug fix releases in the 2.5 branch. I also plan to start working on the 3.x branch, where the first step would be to drop support for TYPO3 CMS 4.5 and move all classes to namespaces, and any other relevant code cleanup. The goal is that version 3.0.0 should be compatible with both TYPO3 CMS 6.2 and 7 LTS. Next would come a 3.1.0 version, which would strictly require TYPO3 CMS 7+. I'm not sure that this scenario is realistic as the backend module is currently completely broken with TYPO3 CMS 7 and I don't know much effort is required to be compatible with both 6.2 and 7.x. Maybe version 3.0.0 will jump straight for 7 LTS.
How and when that happens is however totally uncertain. Currently it mostly depends on what I feel like doing in my free time. I have thought about seeking some sponsorship, but since it won't help me have more free time in my current situation, it's not worth pursuing this idea for now.
The bottom line is that I can't guarantee that External Import will be compatible with TYPO3 CMS 7 by the time it reaches LTS.