I use PortableKeyboardLayout which gives more options and is more lightweight and flexible than an installer. But if you do want to use the MSKLC then you're on the right track. Enable showing all levels of keys, and you'll find the dead key if you haven't seen it already. To the right there's an option to edit it and you'll get a table of triggers and releases for the key. As I recall it you can then either paste (or type) in characters or enter Unicode points for each entry. It's a bit clunky but it works.

If you use my download from the sig topic, there's quite a lot of files there but you should be able to find something useful I hope!

No, the clever programmer Farkas Máté did, but he's from Hungary and doesn't speak much English it seems. Haven't heard from him in a while, sadly.

Caps State, however, is really from MSKLC. Add together a 1 if the 2nd level is the shifted state of the 1st (true for letters but not for number keys for instance), and 4 similarly for the AltGr state. So the CapsState number is either 0, 1, 4 or 5. Get it?

(In case you wonder about the missing second bit: 2 refers to the Ctrl key shift level. As you know, the Ctrl key isn't actually used for shift states so it stays unmapped for the most part. But there's actually one key that's mapped this way: LeftBracket+Ctrl sends an Escape in some layouts, iirc.)

In PKL/Windows, it's known under the name OEM_102 (SC056).

Just look in the existing layout files (proper full mappings, not "VirtualKey" layouts) and hopefully you'll understand how it works. It's based on MSKLC files but instead of codepoint names the dead keys get numbers and instead of code points in hex they're in decimal values.

My own files have AltGr+8 for ogonek ("octonek", haha!) and comma below, as the comma was taken by cedilla in accordance with Combine practice. For a proper Polish layout I did it otherwise of course.

As I said, for proper Polish it belongs more centrally. Yes. But for the full international treatment the cedilla forces it away from the comma.

Using the Wide mod, it sort of gets on the home row! :-)

   
    My Polish Colemak[eD] layout (shown on a Wide modded keyboard)

The German Umlaut is well enough placed for most non-native German usage, at the (AltGr+)semicolon. This is nearly home row. Most combines have it on colon but that's shifted and therefore horrible to use.