They did have them, as major (if rare) status symbols.
Still sounds, at most, like something that would lead to protest. By definition with an Emancipation Proclamation of this type, only slaves of
rebels are freed. (OTL in the ACW, the Union enforced the Fugitive Slave Law in D.C. so long as the owner was pro-Union, for example.)
Still, I wasn't aware native slavery happened at all, so thanks for informing me.
Matters greatly when they do it, who it applies to, the conditions on the to be freed slaves, the POD in Britain that allowed this, how they go about supporting or enforcing it, etc.
The PoD required wouldn't be too extreme - British abolitionism was pretty much first in the world and gathering momentum at this time, and more importantly OTL they did do something sort of like this informally. As for who it applies to, I assume it's presented like the OTL Union one - it applies to slaves of those the British reach who are in rebellion. Stay loyal (and sign an affidavit of loyalty) and you keep your slaves.
If anything this PoD is more about advertising and framing already-extant policy for the British than a radical shift
in policy.