The trickiest part with doing it with the cams out is getting the cams back in. Since some valves will be open, you're going against those valve springs, and not many of the cam caps can be put on to "start" ratcheting the cam down into its saddles. It's a tricky proposition, and it's easy to break a cam cap or crack the camshaft, especially ones that are as long as these. Plus, the engine has to be locked at TDC so it doesn't budge, but you can't lock the cams in the heads and keep them from turning until after they're in. If they turn while you're installing, you could have valves kissing pistons.
These heads/cams suffer from two design deficiencies (IMO, and after working on German and Japanese cars for a "few" years).
1) There's not a portable tool or fixture that fastens to the heads and allows you to lower the cams into their saddles safely. BMW have the same issue, but they have a tool for it.
So after my pre-Christmas whinge about not having a cam installation tool I decided to make one, or actually a couple, because they need to be used, at minimum, in pairs, and the first time I tried them, I used three to make sure the cams were well supported. Here's what they look like in 3D CAD:
And the real thing:
And on the head:
With the clamps in place and snugged down against the camshafts, I then was able to loosen & remove all the cam caps, then I gradually backed off the large nuts on top of the clamps and voila, the cams lifted up just fine. No muss, no fuss, and no danger of the cams "popping". 30 minutes max to setup, remove all the caps, and back off the big nuts and remove both cams.
Unfortunately these clamps won't serve to "lock" the cams, but they certainly simplify the insertion/extraction of them, whether on the bench or with the heads on the engine.
They're workable "as is", but I need to make a couple of refinements in them before I'd let them out of my shop. (For example, the M6 screws at the outer edges really don't do anything. I put them there to reduce the possibility of the clamp "rocking" from one cam to the other, but it appears that's not an issue, so they're unnecessary.)