If you took your average art store transfer and printed on it with plastisol ink and a silk screen then used a heat press on that
Are trying to use the plastisol as opaque under base?
What you need is transfer that release the toner when cold peeled. You probably also need to screen print adhesive over the toner before you screen print plastisol. It may work but the cost will be high due to the cost for screen, transfer paper and labor. It will not be cost effective for one offs.
There is a new low temp self weeding laser transfer that releases the toner. You buy it in pair. It is meant for laser printer with white toner. You discard the opaque part of it. It cost about $3.50 a pair.
Watch this video:
The video shows the transfer is printed with white toner over the color toner , married to opaque transfer, pressed on the shirt at low temp and cold peeled.
You skip the part of marrying the transfer. Instead you screen print the plastisol over the printed image.
If you go that route it is still cheaper than buying laser printer with white toner. The large format printer used in that video costs around $8,000.00. The 5 toner cartridges version is around $19,000.
Water base may work better because plastisol may hot split or not release from the transfer.
Let us know how you are going to do it and post pictures.