FYI - I have not read the article but this is what I would do. Or you could just make a round blank if you wanted. Also, I have never tried to make a mirror so may know all the limitations.
The process would be as follows:
1) Make a positive of the mirror shape out of aluminium or I would just use an existing mirror (scrap preferably) find someone who scratched their mirror and use it.
2) get a flexible bucket large enough to cover the mirror with the plaster
3) Spray the bucket with silicon oil spray as a parting agent.
4) Pour a thin layer of plaster (anhydrite based plaster of Paris) in the bottom of container and let it harden overnight. The plaster should be thin (runny) such that any bubbles can escape (there will be directions on how much water can be added).
5) Put a layer of 2mm poly sheet on the the layer of hardened plaster. Make sure it lays very very flat and extends fully to the sides of the container. The better the fit the easer it will be to separate later.
6) Put the mirror on the poly centred with the parabolic surface facing up. (You may need to coat the mirror in silicon oil but I would try first without as it may make the plaster less likely to work well as the mold).
7) Pour in a thin (runny) plaster again making sure the bubble can rise and cover the mirror with at least 1" of plaster.
8) Allow the plaster to set overnight.
10) Demold from the bucket and separate the upper (flat bottom and mirror from the mold). I am hoping the mirror will slide out, if not the plaster will have to be broken off and a vertical mold split line added)
11) Drill a 1" hole through the flat bottom of mold such that the porcelain or ceramic slip can be poured into the mould.
12) You can sand the mold lightly to remove any unwanted features.
Making green casting.
1) Make the slip. The porcelain slip should be made quite runny to allow the air bubbles to rise. If you are making a slip of another ceramic or glass follow the instructions for the commercial slip solution.
2) Put the Mirror portion of the mold (what was the upper portion before) on a supporting grid to allow airflow under the mould but still support it well.
3) put the top (former bottom) on the mold.
4) pour in the slip through the hole, making sure the bubbles rise out the fill hole.
5) the green mirror blank will be ready to demold when the plaster again appears completely dry (this could take a few days).
6) the blank should slide out of the mold. If it does not allow the open mold with the green mirror blank to dry further (it will shrink more and then fall out) If it does not come out you may have to cut the mold off or a vertical mold split may be needed (vertical poly at 180 degrees appart).
7) Follow the required sintering process in the kiln for the porcelain or ceramic. I would place it with the parabolic side up in the kiln and the bottom on a very flat kiln tile.
1) the end mirror blank will be smaller than the original mirror due to shrinkage of both the slip and further shrinkage on firing. May want to consider using an oversized blank.
2) distortion of the mirror blank can occur in both the green state and kiln. Care is needed in handling and support.
3) the green mirror blank can be sanded/cut as desired taking care not to contaminate it (using files is better than sand paper).
4) Bubbles are your enemy
5)Surfactants can be added to the slip to make bubbles riser better. Surfactants can aid in making the slip runnier.
6) Degassing with a vacuum mixer can help remove bubbles as well (use a vacuum mixer to make the slip).
There would be mistakes and trial and error to get it right but this should work to make a near net shape blank.
FYI - There is no glazing required if the porcelain is fully fired and sintered (same with ceramics). They fully densify. If you do as I have indicated hopefully any bubbles rise away from the surface which will be polished.