Splitting a ticket (on the same carrier) to save almost $2k?
 

Locations involved: Buenos Aires (EZE), Sao Paulo (GRU), London (LHR)
Carrier: Ethiopian (ET)

So R/T EZE to LHR, Business, on ET is $4k+
This flight has a 1:15 stop in GRU outbound, and a 3:20 stop inbound

R/T GRU to LHR, Business, on ET is $1.6k

R/T EZE to GRU, Economy, on ET is $500

These are on the same planes. $4k on a single PNR, $2.1 on separate PNR's. 1:15 is probably not enough time to clear immigration and re-check for the next flight...

Do you think ET would let me check my bags all the way through, and give me a boarding pass in the airside sterile area? Our would they frown upon the obvious ploy to save bux, and strand me?

Anyone pulled this off on another airline?

IMO you should assume that you'll be stranded. Don't forget that you must satisfy all entry requirements for Brazil (visa, COVID-19 test, etc.) in order to be permitted to board the first flight.

OP: Are you able to book the first flight on a different carrier, allowing you to arrive GRU earlier on your day of travel?

Just because your two flights have to be on separate tickets does not necessarily mean that they have to be on separate PNRs. A travel agent might be able to put two tickets on a single PNR. Whether that would affect your ability to through-check your bags is something I do not know.

Some airlines will interline bags on separate flights and some will not. No idea what the ET policy is. But that does not mean staff at outstation know how to do this/will do this.
A map from Great Circle Mapper - Great Circle Mapper

As above would be high risk and require formal entry to Brazil

Different carriers are available, each option comes with its own set of warts (overnight layover, ridiculous pricing, etc.).

That said, the consensus here is steering me to other options.