I will attempt to answer you both, @kenanmike & @jwb, since your questions are somewhat similar.
MetaEdit currently provides one way to do mass editing, which is the context menu which adds a YAML property to files in a folder. This was my first go at adding support, and I have been working on something much grander since. However, that takes a lot of time, because there are many other things that I also need to do to achieve it.
Instead, a fine solution - which works for me - is to use the API and do some scripting.
I use QuickAdd to run my scripts through macros.
I mostly do so on a case-by-case basis, but try to abstract over the specific needs if it makes sense to do so. This also allows me to share scripts which might serve as inspiration:
For mass editing, the general idea is to get all the files you wish to target (through the Obsidian API), and then using the MetaEdit API update function on them.
I hope this helps 