Can I use a *ground wire only* conduit for copper data cables?
What do you mean by “retrofit ground?” This phrase makes no sense to me.
If you have a 3 wire feeder (L1, L2 and a “grounded” conductor, aka a neutral) you dont have a ground at the shed and one must be established using one of the approved methods found in article 250 of NFPA 70 (assuming you’re in the USA). You reference 1999, which is a NEC code cycle year in which 3 wire feeders were probably permitted.
If you have a 3 wire feeder to the shed you would have had to install... ground rods or a UFER etc..   The neutral and ground bus bars in the panel would have to be bonded together and you would be establishing a new ground at the shed.
With a feeder, it is required in NFPA 70 that the grounding conductor/ ground wire be ran/routed with the ungrounded and grounded conductors. See Articles 250.24(c) or 250.118 or 250.186(A) and/or 300.3(b) of the NEC.
You cannot just throw a ridged metal conduit in the ground and bond it between your source and your shed and now say its grounded. The ground path must be routed with the current carrying conductors and be installed in a very particular manner.
Your second idea about using PVC with a number 2Al also doesn’t meet the requirement that the conductors all be routed together.
If you install a second conductor between the source and the shed. This second conductor needs to be treated differently from the grounded (neutral) conductor because it isn’t a current carrying conductor (it isn’t meant to carry the unbalance neutral current, its purpose is to facilitate the operation of the over current protection device (breaker) at the source.
Bottom line: if you want to “retrofit” your ground you will need to pull a ground”ing” wire in the same raceway/pathway as the other 3 current carrying conductors. You will need to separate the grounds and neutrals at the shed so that you dont have parallel paths to ground. And deal appropriately with any and all parallel metal paths between the source and the shed.
Again, check article 250 for requirements on grounding and article 230 for feeder requirements.