Brett, i didn't specifically say ringworm (i'm thinking yours is a "typo" and you meant to say "roundworm"). I was thinking more of tapes & flukes, but any larger parasite, like nematodes would probably fall into the same category is my semi-educated guess.
[BTW, "ringworm" is NOT a type of "worm" - it's actually a fungus, NOT a roundworm - generally one of two species of dermatophyte fungi similar to that causing "Atheletes foot" & "jock rash/itch", etc.; BTW, Jewelweed aka Touch-me-Not, properly prepared, is a great natural remedy for "ringworm" as well as for poison ivy - works GREAT, IME]
As far as an UL solution, hmm..., YOUR coffee filter is pretty UL. Yes, it's Mfr'd w/an uncontrolled pore size, but supposedly your typical paper coffee filter has pores of varying sizes ranging from b/t 5-15 microns inclusive (that's why they filter so slowly!!). This should handle most (all???) of the lil' nasties not rendered lame or dead by UV-C or chems (at the normal L/UL backpacker dosing levels - higher levels can kill them, like ~24x to ~28x higher UV-C levels will eradicate ~98% of hydatid tape "eggs").
"No", a paper coffee filer is NOT too robust, so i don't recommend them for inline gravity (unless examined b/f each use) or on-demand inline filtration, but for prefiltering where its integrity can be examined (yeah, i know we'd need something like a stereoscopic dissecting microscope, or better, to see larger than the 25micron perforations that could allow tape eggs to pass), i'd, personally, go with it.
Unless one is in an indigenous area where these rare nasties are known to exist, it's generally considered something that one needn't worry about. Theoretically, though, they could be most anywhere certain types of host creatures are located (e.g. wild or feral canines & hydatid tapes).
The way i look at it is, if some of these rare nasties were that much of a problem (outside of KNOWN indigenous areas), we would already have a "TON", so to speak, of ANECDOTAL evidence from the medical problems of the many, many UL Thru-Hikers who don't treat for these nasties using normal pump filtration (more commonly used by heavier-wt backpackers). In other words, we'd have common knowledge shared by others who Thru-Hiked and said something like "50% of us are coming down with this...or that...clinical disorder/disease/illness, etc." Now, some of these may not show up right away, but i think by now, decades later that there would be some anecdotal evidence if this was a problem. Instead, what do we hear about?, viz. Giardia, Crypto, etc. - NOT flukes, tapes (other than indigenous areas, e.g. Isle Royale NP), roundworms, etc. Sure, unlike with smaller buggers, it might only take ONE ingested egg or [meta]cercariae (depending upon the parasite involved) to cause a problem (smaller buggers, e.g. bacteria & viruses, usually require a much larger "load" [a real term, not a "pj-ism"]). However, to my way of thinking, the fact that even ONE can cause a problem, only supports my reasoning that anecdotally we'd already "KNOW" that these larger, rare nasties are something we should be concerned about. By this i mean, generally "odds" are easier of encountering ONE rare occurence than a whole bunch of them.
Are these rantings clear? or, have i "muddied" the waters, so to speak?