...unless I can find some factory made stuff in the interim.
I'm thinking you might also be able to make some fairly uncomplicated subsonic loads using either Trail Boss or even Blackhorn 209. 
The only source for factory loaded subsonic shotgun slugs, I am aware of, is:
https://detroitammoco.com/product/subsonic-12-gauge-2-12-1-18-oz-slug-box-of-5/------------------
Trail Boss is an extremely "fluffy" fast burning smokeless that does not work well under compression.
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Blackhorn 209 is essentially a
bulk smokeless powder* so it would work abeit at greater expense than other pressure tested
dense smokeless powder* options.
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*Thes terms are somewhat obsolescent but properly describe the difference in suitable powder types.