Two other IPs have relayed new empty blocks (and have not relayed non-empty blocks) in recent days, including another two in a row yesterday:
http://blockchain.info/block-height/171806 (relayed by 188.127.227.12)
http://blockchain.info/block-height/171807 (relayed by 213.171.43.151)
Here are blockchain.info's lists of transactions first relayed by each of the four IPs I've seen relaying new empty blocks recently:
http://blockchain.info/ip-address/88.6.216.9 (29 empty blocks between March 3 and March 7)
http://blockchain.info/ip-address/85.214.124.168 (74 empty blocks between March 15 and March 19)
http://blockchain.info/ip-address/213.171.43.151 (9 empty blocks between March 14 and March 19)
http://blockchain.info/ip-address/188.127.227.12 (6 empty blocks between March 16 and March 19)
All of these IPs have also relayed other transactions, which leads me to this theory: Perhaps these IPs are just regular bitcoin nodes, and are not related to the empty-block miner at all? They could just be relaying transactions and blocks for everyone, and the empty-block miner is merely choosing for some reason to always relay their work through this small set of nodes.
It would be easy enough to test this theory (or confirm its negative it, at least) by trying to relay some transactions through them, but I haven't done that. I did however confirm that two of them (85.214.124.168 and 213.171.43.151) are currently listening on the default bitcoin port (8333).
Also, to repeat my earlier question, could there be any significance to the out-of-order timestamps in blocks 171759 and 171760 other than indicating that the empty-block miner's nodes don't have synchronized clocks?