Well, I tried what you outlined here. I definitely misunderstood the instructions first time around. This time, I was able to edit the font stack for Segoe UI, and the loaded Japanese font did in fact change. Here is the registry value and result.
ShipporiGothicB2-Bold.ttf,Shippori Gothic B2 Bold
MEIRYO.TTC,Meiryo UI,128,96
MEIRYO.TTC,Meiryo UI
MSGOTHIC.TTC,MS UI Gothic
MSJH.TTC,Microsoft JhengHei UI,128,96
MSJH.TTC,Microsoft JhengHei UI
MSYH.TTC,Microsoft YaHei UI,128,96
MSYH.TTC,Microsoft YaHei UI
MALGUN.TTF,Malgun Gothic,128,96
MALGUN.TTF,Malgun Gothic
MINGLIU.TTC,PMingLiU
SIMSUN.TTC,SimSun
GULIM.TTC,Gulim
YUGOTHM.TTC,Yu Gothic UI,128,96
YUGOTHM.TTC,Yu Gothic UI
Font Awesome 6 Free-Regular-400.otf,Font Awesome 6 Free Regular
Font Awesome 6 Brands-Regular-400.otf,Font Awesome 6 Brands Regular
FONTAWESOME-WEBFONT.TTF,FontAwesome
SEGUISYM.TTF,Segoe UI Symbol
SEGUIEMJ.TTF,Segoe UI Emoji
SEGMDL2.TTF,Segoe MDL2 Assets
SEGFI.TTF,Segoe Fluent Icons
TAHOMA.TTF,Tahoma
ARIAL-UNICODE-MS.TTF,Arial Unicode MS

Unfortunately, you might be able to tell that the Playlist View is still rendering fonts differently. I presume that means that it has its own font fallback mechanism separate from the other components (why???).
If anyone knows what the hell is going on here, that would be great. I haven't had the time to learn about making JSplitter/SMP's work on Linux yet in general, much less so a custom playlist viewer. I presume I might not be facing that issue if I just switched to something else.
I was able to get proper font fallback working for Segoe UI, but not for Lexend Deca. Here are my results. Lexend Deca shows up as such in foobar2000, so I named the registry key that. It has the exact same value as the Segoe UI one. Is this the correct method?

Didn't work again for playlist view; Japanese text is obviously not Shippori Gothic B2 Bold, but rather Source Han Sans.
Any help or testing ideas would be appreciated. In the meantime, I might just look into switching playlist viewers if I can get it in everything but that. Or switching to Default UI. Also thanks for the help so far @paregistrase.
Additionally, trying to replace Segoe UI with Lexend via registry as I outlined above did not work at all.