Tried that. With and without heat. Broke the EZ out off in the drilled hole.
Rule number 27 (I just made that up) , never try to remove a bolt that broke off due to corrosion with an EZ out, broke off going in yes, no problem. Not a very useful comment I grant you at this point, that horse is out of the barn but keep it in mind.
You need to make a small drilling jig, no need to buy one use your manifold as a template on 3/8 flat stock (aluminum is fine) with a drill press. once you have the holes drilled for the correct size bolts eg M6-8 drill the holes in the jig half way though only with a larger size drill, more on that later.
Get some round stock about 10-12 mm O.D. 10 for M6 or 12mm for M8 bolts and cut it into 1/4" and one 1" lengths (you may find washers you can use for the 1/4" ones. Depending on how many bolts the manifold uses for an example we will use 6, drill 5 of these through the middle with the original bolt hole size and the 1" long with a M3 or M4 hole.
The O.D. of these slugs is the size you will drill in the jig plate halfway through. Carefully grind any protruding bolt or tooling flush with a dremel, bolt the plate with the slugs using the smaller slugs and place the long one over the broken bolt by switching the location of the slugs any of the holes can be drilled through the longer one.
The small hole will act as a guide, in the hole with the broken EZ out in it you want to use a right hand drill bit, it will eventually loosen, once it is out go to left hand bits.
Use you original bolts to determine the depth of the drill bit including the slug and put a few turns of tape or drill stop at that point so it cant go past the slug.
Aluminum drills very easy and is easy to work with, for a longer lasting tool I would make the long slug out of steel but its not necessary for a one off job with a few holes.
Once the hole is drilled you can work up on size at a time without the slug, the bit with chase the hole by itself.
I am no expert but this has worked for me many times.