We've been using AC3D in our teaching for many years now and (being mainly concerned with real-time defence visual apps) have also occasionally searched for OpenFlight modellers. Multigen Creator itself of course being completely out of our league for multi-seat student use in our lab (even at academic prices). We do use Carbongraphics' Geo modeller, which works OK and both imports and exports OpenFlight, and which is more reasonably priced. But its still in a different league (price wise) to AC3D I'm afraid though. Even the converters that deal reasonably well with OpenFlight (like Polytrans) aren't cheap. Right Hemisphere had a package that undestood FLT a few years back (3D Exploration) and which was modestly priced, but its now been re-packaged into 'Deep Exploration' that isn't so modestly priced...
The freeware PrettyPoly modeller (
http://prettypoly.sourceforge.net/) claims to deal with FLT, but it hasn't been updated in several years now, and we've never had much luck with it for anything serious.
Bottom line, I'm afraid I've never come across a good low-cost OpenFlight modeller, although if you find one, let me know! Our staff have therefore ended up using Geo to build OpenFlight models when we need to, but our students themselves use AC3D to learn about modelling. Our visual apps use OpenSceneGraph at present, which can then import both .AC format and .FLT format.
Albeit AC3D isn't quite good enough yet at model hierarchy organisation & grouping for real-time usage and doesn't understand anything at all about things like LevelOfDetail, DegreeOfFreedom and Switch nodes.
Hence we've ended up creating another tool (using OSG) to take AC models and modify the nodes and hierarchy for real-time use. Even that special tool doesn't create .FLT however, it only creates OSG's .OSG (text) and .IVE (binary) formats, as OSG has a FLT loader but not a FLT writer.
Given the effort that we saw go into the Geo OpenFlight loader and writer during its beta programme, it certainly doesn't appear to be a straightforward task to deal with all the complexity of the OpenFlight format in all its various revisions. I've written simple parsers for it (eg to change texture refs etc) but not whole loaders, converets or writers.
Sorry not to be able to offer a more optimistic outlook, hope that helps!
Jonathan