How/where do I publish a 50-page-long writeup?
The options are really to either find a place that will publish in the format that it is or to figure out how to break it up.

The first thing I would do is try and get an accurate estimate of the length. A 50 page document with the standard LaTeX font and margins that includes a TOC, index and big diagrams may not actually be too long for a lot of journals. Depending on the complexity of the article, you can try creating a "preprint" to get a pretty good approximation of length. A 20 page article can probably be made to stay under the maximum length requirements of many journals. Getting a 40 page article to work will likely not be possible.

In terms of breaking it up, you need to figure out how to chunk the work.


  Either it would have no grounding and be based on hand-waving; or it would be bits-and-pieces all over the place; or it would be a long series of definitions which doesn't go somewhere very interesting.


Trying to publish bits-and-pieces all over the place is going to be painful. Most reviewers want some sort of cohesive story. The other two options sound like dividing the work into two chunks. The first chunk seems like something you could publish in a good journal if somehow the theoretical grounding was published.

The second chunk seems boring that no good journal is going to want to publish. I see three approaches to getting it published. The first is to publish the second chunk as a stand alone item. Depending on your field you might be able to publish it as a methods article or in a methods journal. You could also target a low impact journal/conference that is targeting work that is slightly better than pay-to-publish quality and doesn't really care if things are interesting. Finally, you could use something like ArXiv (or whatever your field uses) to self publish the work.

The second option is to publish the second chunk as supplemental material in a journal that allows that. Here is an example of what APS supplemental material can include.

The final option, and probably the most difficult, is to find a journal that allows big appendices.