I've found that Lenovo ThinkPad adapter/converter, part number (FRU, Field Replaceable Unit) 42W7888, is a common and thus cheap pass-through-only adapter for 1.8" micro SATA to 2.5" SATA. There are plenty on eBay for around 4-8 USD.
I also looked for adapters which do come with voltage regulators, seeing as regular many-2.5"-bays-in-one-5.25"-slot enclosures that I find do not appear to supply 3.3V. At least that is my guess for those enclosures which use regular 4-pin molex connectors, that supply 5V and 12V only. There are a few exceptions, e.g. from Icy Dock, which accepts SATA power connectors, and hopefully passes through all power lines.
The only adapter I've found pictures of with a clearly visible part number on the voltage regulator shows an AMS1117 3.3, which is rated for only 1.5A. SATA spec says each voltage pin should be able to supply 1.5A, of which there are two in the micro SATA connector.
The drive I have in mind is the Intel DC S3700 400GB 1.8", which is quite power hungry. It has a typical draw of 6W when active, and can peak at over 7W. This is more than this single AMS1117 3.3 will be able to handle, so those adapters are non-options.
Icy Dock ToughArmor MB992SK-B, to put two 2.5" drives in an external 3.5" bay, is interesting. However I'm afraid it may not be providing sufficient heat dissipation when stocked with two of the S3700, as there is no active cooling.
