The idea of putting a plug into someone who has had a suppository is interesting, partly because it will help the recipient hold the suppository longer hence increase it's laxative. A good plug will also reduce possible premature leakage.
But I don't think a feminine hygiene tampon is the right product to use. For one thing, they are designed to absorb, rather than to plug the passage. And a med absorbed into the tampon isn't in contact with the bowel wall so isn't working as a contact laxative.
IMO, a better choice would be something like a menstrual cup. There used to be disposable ones, called the Tassaway, but it's gone I think.
http://www.mum.org/CupTsway.htm
There are similar products on the market now, made of silicone which would be interesting to try.
There are also products for those who ae poo incontinent, made out of a soft foam, that are designed to be inserted rectally and pass gas but not liquid, called Peristeen: Anal Plug:
https://products.coloplast.co.uk/coloplast/continence-care/peristeen/peristeen-anal-plug/
https://ibb.co/hFP5t3f
IMO, this would make a much better plug to inset after a suppository o two.