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AFAIK, only the (very) old ICH7R (or ICH8R, it has been a while since I last checked this) supported port multipliers. All other chipsets either don't support it or the manufacturers don't present it as a feature basically ANYWHERE, so there is no way to know for sure short of hands-on experience...
I do think, though, that chipset manufacturers are not very interested in PM support on consumer level motherboards, so my first guess would be don't count on it.
A recent search I made, though, taught me that right now there are two storage controller manufacturers (for low-cost options, of course) that handle PMs: SiI and JMicron. For SiI-based PM support, check for the 3132 (PCIe 1x to 2x SATA/eSATA) or 3124 (PCI to 4xSATA/eSATA) chips; for JMicron-based PM support, I think that just about everything including and after JMB363 (VERY common on motherboards since the Intel P series chipsets launched) support PMs.
From what I can tell, you can either have one of those, not both. You can try something with the 8200/9300 chipset (AMD/Intel), but you'll have to add a storage controller on the PCI/PCIe slot (and pray the PCIe slot recognizes it and doesn't shut down the IGP, which sometimes happens).
Thanks Miguel for the really helpful post. I may go with the ITX-9300 and a JMB360 PCI-e 1x eSATA. Only 14 bucks so it's worth a shot. I will probably saw off the extra metal as my build is very compact (which is why I need PM)