I understand how to use proxies in the normal way, but this is different.
Normally, you start with a hi-res footage and you make/set a proxy of a low-res version for faster processing. I get that.
If you *start* with lo-rez, though, your footage will usually be scaled way up. If you try to trick AE my setting a proxy using the high-rez file, it’s pixelated because it tries to “preserve” the scaling of the original (lo rez in this case) and then reduces it to fit the original dimensions.