I’m author of that gist (just noticed I have some comments there I need to reply to), still running the setup, now with FreeBSD 12-CURRENT. Tl;dr it generally works, still no wi-fi support. X works fine (running with drm-next-4.7 patches to get external thunderbolt display to work; vanilla kernel works fine with built-in and DVI display, gets confused by Thunderbolt display only). Gave up on pkg because I want to control my build flags, I’m using portmaster now and thinking about setting up poudriere on a bigger server. Feel free to ask if you have any particular questions.
Broadcom wifi support for *BSD is limited to older chipsets, I’m afraid.
DragonflyBSD originated but has now removed bwi(4), the driver for powerbook G4 era wifi chips.
FreeBSD has kept bwi(4).
FreeBSD developed bwn(4) for newer chips but it does not seem to support the very latest chips either. Perhaps that will change some day.
This driver used to “not work (don’t bother porting it to OpenBSD)” last I asked, but Adrian has come up with some fixes for it since.
OpenBSD still has bwi(4) but no equivalent for bwn(4).
Your best bet is probably to use a USB dongle supported by run(4) (Ralink/Mediatek chips).
There’s also urtwn(4) but the hardware tends to overheat and the quality of the driver is worse.