Firstly, I don't intend this post as a "hurry up" or "how's the status", just to be clear. This is an overview of my findings from a long series of tests and a substantial amount of research.
I am still very much in support of MNG being added to FSO still, but I do have a lot of concerns about the format. It has a lot of positive sides to it. MNGs, in my tests look really great and in many cases they are 50% smaller in file size than EFF sequences.

All in all, I am very happy with the format...
When it works...
As discussed
here there
are were at one point many tools for creating MNGs and a few for playing them back. And therein lies the problem. MNG, while a great format, may as well be considered a dead format as for as the general populace goes. When MNG died, so did most of it's tools. Many of them never really got off the ground enough to be even considered in the realm of reliable. Trust me, I tried every single one I could find. I did all this testing because I have hundreds of ANIs to test it on, and to save you guys the trouble.
However, the problem gets more complicated because there are even fewer MNG players out there. I tried as many as I could find as well. The end result showed varying degrees of success. Some MNG files would work in players but fail in others. Programs that should seemingly have reliable support (Like IrfanView) didn't work at all for any MNG file I could throw at it.
Eventually in my testing, I landed on MNGSV as my player of choice. It successfully played back the most MNG files I could throw at it, 6/10 in my tests. None of the others came close. IrfanView was an incredible 0/10. So, with my player chosen, I began a thorough search of an MNG builder and finally landed on AdvanceCOMP. Using MNGSV as my playback tester, I tested each builder with the same sporadic grouping of my FS1 and FS2 ANIs. AdvanceCOMP scored the best with about a 45% success ratio.

All that to say that there isn't much support out there anymore for MNG files. Both MNGSV and AdvanceCOMP seem completely ignored these days as far as development. If we are going to get MNG support, then I want to stress that we are going to need reliable tools as well. As it is, I can't really tell if it's AdvanceCOMP or MNGSV that is failing. It should also be considered that maybe the MNG libraries are unreliable. But that's getting out of my realm of knowledge.
At various times I thought certain things were hindering the creation, like having capital letters in the name or something. Finally though, I just started creating MNGs from PNG sequences labeled ####.png. Still no good. I tried different levels of compression, different types of PNG files. In the end, I never could tell if/why one would play and another wouldn't...
Finally, I'm linking to the download pages of
MNGSV and
AdvanceCOMP as both of them also have the source up for download. Perhaps that could be a start to making reliable tools? I'm also linking to a few MNG files I've already created. There are two large MNGs and two small MNGs, one of each works and one doesn't. Figured that would help with any testing that might go on.
Non-Working MNGsWorking MNGs