I don't see any reason why not, assuming you have (or can practice to develop) enough skill with the micrometer to generate repeatable measurements. Micrometers have a "feel" for when they are properly in tension on the part-- the very flat surface finish of the anvils will actually create surface tension against the part and "stick" the anvil to it.
You can practice this by using a micrometer standard. A "standard" is a piece of steel of known length, say 75mm, which also has flat ends. This is inserted between the anvils and the mic is tightened down using the ratcheting feature of the mic-- setting it at exactly 75mm. Then you use the little wrench that comes with the mic to zero it out. Anyway you can practice repeating the 75mm measurement over and over and over until you can do it with your eyes closed. A light oil on the standard and the anvils will help with the feel. Wipe off the instrument with iso alcohol when you are finished and put some light oil on the anvils and standard faces or it WILL rust and throw off your measurements.
There are inexpensive digital mics available, they switch between metric and decimal-inch, whichever you prefer.
My ARP bolts had a little divot in the end, seems to me that a micrometer with a ball bearing anvil attachment could fit repeatably into the divot, then you could use the conventional anvil on the other end. Rather than buying a ball micrometer (which has a ball end on the anvil permanently) you can buy accessory anvil tips (ball, flat, point etc) for not too much money. The key is that you have to subtract the length of the tips and they must absolutely fit the same way every time.
So it's not impossible and mic's are used in aerospace frequently to determine preload.
It's also a good idea to measure the new length of the bolts. That way if one deviates significantly from the others, this is a red flag that there may be a manufacturing defect. Not likely, but still possible and easy to check. Find a way of marking all 12 bolts, then create a log of the unstretched length. That way when you tear down later, you can tell if one or more has permanently deformed. If I remember right ARP says that if a fastener has stretched permanently more than 0.001" it's considered junk.
Anyway, the procedure would be to put the bolts in, turn the nuts finger tight, take the measurement and then do your first torque to a reasonable torque level, say 25 ft-pounds. Before the pundits jump on me here, I'm not suggesting you use the torque method, what I am saying is you want to creep up on the correct stretch, so just randomly reefing on a breaker bar could cause you to overshoot the correct preload. Make a stretch measurement, write it down, then subtract the "new" length to get a sense of how far you have to go.
Good luck! Ask me the time I give you instructions on how to build a swiss watch. . .
