Pediococcus contamination--Will brett get rid of the ropiness?

Without a photo, it sounds like you have the makings of a pellicle, although the statement "a thick ropiness below the surface" is a bit confusing. Pellicles form on top of the beer, and have the appearance of anything from a slightly translucent film to what looks like a long-lasting, inanimate krausen. Sometimes people use the term ropiness to describe a particular appearance of the pellicle.

If it is a pellicle (we'll be able to better diagnose with a photo), Brettanomyces will not get rid of the pellicle. Sometimes the pellicle will fall back into the beer with enough time, other times you'll just have to stick your racking cane into it. It's nothing bad, and won't hurt you/your beer. It's really not possible to easily say "that's a Lactobacillus pellicle" versus "that's a Pediococcus pellicle". Chances are, it could be both, or something else altogether such as acetobacter.

That said, if you do have a Pediococcus contamination, Brettanomyces is a necessity. Pediococcus can make your beer go sick, and produce Diacetyl. By "sick", I mean it'll turn viscous and taste real slick and disgusting on your tongue. Brett will clear that up with time. According to what I've heard, famous sour beer brewers such as Cantillon and Russian River have all recovered sick beers by using Brett. If they can produce world class beers that became sick at some point, it's good enough for me.

Diacetyl is another flavor compound Pedio can produce. Best description is buttery. Brett can clear that up too, but it does take a significant amount of time. One very important thing to consider when pitching Brett, is that you will need to extend your aging to the upwards of a year or more, or you can risk having over-carbonated bottle bombs. Reason for this is Brett is a very resilient yeast. It'll absorb all the oxygen it can and keep fermenting. Supposedly people have discovered brett in 30 year old beers that have survived off whatever little oxygen it can scavenge while in the bottle. Also, as saccharomyces dies off and self-digests (autolysis), it will split open and release fermentable sugars that the brett will ferment out, as well as fatty acids that it will convert into really interesting ester flavor profiles. If you plan to pitch brett, commit to ageing the beer for a good amount of time.

An unorthodox (by today's standards) way to deal with it is the really old school way of using mustard seed.

When beer turns ropy without being sour, it is easily restored by mixing in the proportion of one spoonful of mustard to every fourteen gallons, in a little of the beer, and pouring it into the bung-hole. In the course of the next day the beer will be fit for use. When it is actually sour it may be restored by hanging a linen bag in the cask, with equal quantities of pounded chalk and calcined oyster-shells. This will cure it in the space of a day and a night, but it will not keep very long after these additions.

I find that interesting what it says to do when it does go sour. It sounds like they added alkalinity to counter the acidity. I wonder if the reason it doesn't keep long is that you've raised the pH and it is now a more hospitable environment for spoiling organisms?

Another reference I found says half a pound of mustard seed to a hogshead (which is somewhere around 50 gallons), so 1.6 oz per 5 gallon batch, and it will be good to go in a week or two.

Though unlikely, if one were to try this route, I wouldn't use ground mustard seed for fear of adding too much flavor. Everything I've seen in a number of 19th century brewing books says either whole seed or "lightly bruised".