For those not of the Imperial persuasion, it really isn't so much about how the person became a slave that is abhorrent. A system where citizens are programmed from birth via Imperial cultural propaganda to turn themselves into slaves cannot be said to be much of an improvement over old-fashioned kidnapping and slave raiding. It is what happens to said slaves afterwards that is abhorrent.
Slaves, of both flavours, are commoditized people. You speak of "protections" offered to Imperial Slaves, but such protections are invisible and irrelevant to us, as the potential slave purchasers and transporters. Have you ever bought an Imperial Slave, only to have that slave decline your offer to transport them off-world? No, of course not. They are fungible cargo. And while some - the ones found in mission transactions - have some guarantee as to where they specifically will end up, those who are traded on the open market have no such guarantees. The only thing preventing a pilot from selling the Imperial Slaves they buy to Archon Delaine is the honour of the pilot in question - and if that pilot is not RPing a loyal servant of the Empire, then that honour is nonexistent. We don't even need a Imperial slavery shipping permit.
As for the eventual fate of the slaves that don't get exported out of the Empire, let's see what ED itself has to say about them. Open the galaxy map and click on one of the older Imperial mining colonies - any of the ones that were in the old FE2/FFE games and therefore had their system descriptions copied over from there into ED. Any one of them will do, but if you're having difficulty finding one, click on Laedla. Read about how the system's economy requires the use of slaves to maximize the efficiency of ore extraction. Now find an "old" agricultural world; Cemiess is nearby to Laedla; the first paragraph was copied over from FE2/FFE. Again, read how Imperial Slaves are seen as necessary to keep the economy churning over. Such slaves may be worked to death in these environments, just like "regular slaves" would be outside of the Empire, unless they are strong, or lucky, or manage to get themselves sold into a cushy job that doesn't involve hard labour.
Changing the label will change none of this. And will not make the concept any more palatable to non-Imperials.