Now I’m able to answer my own question, after seemingly having gone down one rabbit-hole and come up through another.
As the conversation in the comments on the question goes, it should probably be possible to place iPXE in an Apple Disk Image inside a NetBoot Image and chainload into it using BSDP. However, as always, the devil is in the details and with my specific hardware (MacBookPro11,1; A1502 EMC 2678) this idea is not feasible to pursuit.
The weakest link is the wifi interface, the Broadcom bcm4360 with pci id 14e4:43a0. These cards are not like the bcm44xx series. As a matter of fact, they are not even comparable to other bcm43xx cards. The only datasheet one can ever find for them, at least without signing an NDA, is a thin eight page document describing trivial things such as its physical dimensions. Unlike the similarly named cards, no one has published any open-source drivers. To the best of my understanding the wifi is almost completely unusable with FreeBSD, illumos, NetBSD and OpenBSD. They only work under Linux if loading binary blobs provided by Broadcom (alpine, arch, debian, ubuntu). One can safely assume that iPXE will not implement drivers for a device none of the open-source operating systems support natively.
With no shortage of voices believing facts to be different, the best place to look is the actual source codes. The PCI_DEVID_BCM4360_D11AC constant is not used anywhere in FreeBSD, NetBSD completely lacks the expected PCI_PRODUCT_BROADCOM_BCM4360 constant, and OpenBSD does not mention the PCI_PRODUCT_BROADCOM_BCM4360 constant when enumerating the list of supported devices.
When I was given this computer a fews days back, I believed one could run an open-source Darwin on it. It seems however that OpenDarwin was announced dead in 2006, and PureDarwin lost traction around 2015. Even if there were active communities, Apple would have been contractually prohibited to open-source their wifi-drivers, effectively making wifi-support macOS specific rather available in Darwin¹.
Supposedly there are macbooks with other wifi chipsets. I’ll leave my own answer unaccepted, hoping someone will eventually provide an answer which actually gives a full solution to the desired use-case for anyone with a supported card.