Just after the DTV transition there was a proposal forwarded for "Digital On-Channel Repeaters" but I expect that they wouldn't qualify as SFN as they aren't precisely phased.
From what I read in Doug Lung's 2009 proposal, the Digital On-Channel Repeater (DOCR) described was phased. I'm not seeing much conceptual difference, if any, between the DOCR proposal, the Digital Transmission System (DTS) approved by the FCC, and the SFN proposed in ATSC 3.0. All were meant to provide fill-in coverage on the same channel as the primary channel, and all were supposed to be synchronized so there would be virtually no interference to other transmission sites. The only major difference seems to be in the planning and licensing.
This impresses me as another technology that would put the hurt on those with DX setups as you would be able to pick up many "cell's" signals
Unfortunately for those who enjoy DXing, the ability to pick up distant signals is of no concern to the FCC or to the TV stations.