OK...this may be convoluted, so please bare with me...I have a mixture of different plumbing materials in my house. It is a half a double with a shared sewer line. I recently had a water fall in my kitchen because the copper drain pipe from my tub backed up and let loose dumping a half a tub's worth of water into my kitchen below. upon opening up the plaster ceiling, I traced the drain pipe from the tub/sink to a cast iron elbow from the toilet. My problem is, I recently had to change the main cast Iron pipe in my basement over to PVC because it was failing and leaking at several places. I replaced from the vertical that drops between the two houses. OK...my problem is, I cant replace the elbow to my toilet because it is a street elbow that goes directly into a "Y" and drops down between the two houses...sandwiched between the block dividing wall. The toilet elbow looks OK...some surface rust...but as my experience with the horizontal waste pipe showed me, I cannot tell the condition of the inner walls. Is there any type of product that I can coat the outside of the elbow that would protect it from rusting through? almost create a shell that would be there in case it rusts through from the inside? The toilet is right over my pantry and I cannot afford to lose the things in that closet.

if it looks OK with just some surface rust i would not worry about it. cast iron does not simply colapse. it will develop pin holes in the casting or it will develop a crack along a seam but it will not explode and cause a flood.
if the pipe is above a ceiling in a pantry, just put an access panel that you can open once a year to check the pipe.

I have seen occassional cast iron pipe failures (pinholes, seam splits,etc.) but don't recall having ever seen a fitting failure.