UPDATE 19 MARCH 2016
Got my U.FL to RP-SMA pigtails with antennas. I have a few nice black cases by OneNineDesign that came with Rapsberry Pi 2 kits by CanaKit. I modified one for the Pi3 plus pigtail & external antenna.
I cut a little notch in the plastic for the wire to come up from underside the board, hollowed the Pi2 LED holes to pass the cable thru, then drilled a hole above the power connector to mount the SMA connector.
Then finally I mis-measured & ended up with a terrible hole for the Pi3 LEDs to shine thru. :P
See additional link to Google Photos album called Case Modification.
With my Fluke multimeter I checked continuity from the radio antenna output thru the two tiny zero-ohm resistors, all the way to the metallic looking parts of the onboard antenna chip. I got beeps all the way.
I checked continuity from the radio antenna output to the little gold square where the U.FL center pin contact might go - nada, open.
Alright, time to potentially ruin my brand new Pi 3!
I imaged an SD card with Raspbian Jessie Lite, setup everything to my preferences, and installed wavemon. After each step of abusing my Pi 3's circuit board, I let it cool, inserted the SD card & powered up, SSH'd in, checked wavemon, then took photos.
I have linked to my Google Photos album so you can see all the pics I took. For the pics I've uploaded to this project, I used Skitch to screenshot & markup with the arrows and text.
There is ground plane on the opposite side of the exposed J13 rectangular pad, so I scraped off some solder mask, then I tinned up all three spots.
I took a wifi module out of a junk laptop then with my heat gun and tweezers I lifted both U.FL connectors from it.
Heat gun and tweezers again, I placed the U.FL connector on J13.
I snapped on an antenna lead I pulled out of the same junk laptop - oops, when it swivels it will short the Pi3's ground plane to GPIO pin 1 - 3.3Vdc. I mixed up a little bit of non-conductive epoxy and put a blob over that pin to insulate it - problem solved.
I put a tiny piece of solder at the radio's antenna output & the dot where the U.FL square pad trace comes up, melted it with the heat gun. Check continuity again with the meter - YAY! Got continuity with both the onboard antenna chip and the U.FL center pin.
I booted it up and ran wavemon to check signal strengths with & without the laptop antenna lead connected, not much difference.
So now everything still works, time to go all in! I fired up the heat gun again, with a needle pick I flicked away the zero ohm resistors (I have no clue where they went, so tiny!) that run along the little J-shaped trace thru the via to the onboard antenna.
I checked continuity again - onboard antenna nada, U.FL center pin game on!
Finally I booted up without the laptop antenna lead. I was able to SSH in and run wavemon, my signal strength to the AP 10 feet away was -66 dBm, and my neighbors didn't even show up on F3-scan. I snapped on the laptop antenna lead, and BLAM! My wifi was -29 dBm, and the neighbors showed up in wavemon as I expected.
With this modification a Pi3 can easily mount inside a metal enclosure. My omni antennas with U.FL to RP-SMA pigtails will arrive tomorrow. =)