The Prostate contributes only 25-30% of ejaculate, with most coming from the seminal vesicles. So I wonder if those focussing on prostate massage/stimulation are missing the point here? Likewise to establish fertility, the 2-5% from the testicles would need to be present.
There are techniques to extract semen from paralysed men that use an electrode up their rectum, but it seems this is far from pleasant and requires sedation or general anaesthesia in a hospital setting. Some who have obtained such machines do this as part of BDSM play.
There's a useful post in a similar thread here:
https://en.zity.biz/index.php?mx=forum;ox=display;msg=1273442
Here is one hospital machine, note the temperature control to avoid damage:
https://www.researchgate.net/figure/Electroejaculation-machine-with-an-attached-rectal-probe_fig1_298733369
Meanwhile for those who can ejaculate but who feel shy about the traditional 'magazine & plastic jar' method, the Chinese are releasing these 'Sperm Extractors' to hospitals. Apparently using one of these is supposed to be less embarrassing....
https://nextshark.com/china-sperm-extractor
Watching the video, it's hard not to imagine these won't be sold like domestic appliances soon enough.