Although I heard that such Series IR filters might have been made, after searching a long time I never did fine one. Consequently, I can't say for sure whether there really are or aren't any out there. But in the mean time I came up with what I think is a really nifty DIY solution; something you might consider yourself. To make a long story short, have a look at this "Series-VI" IR filter, which I put together myself:
finished filter.jpg

If you look closely, you can see that the body of the filter is actually just a Series VI retaining ring - the double-threaded type to be exact, i.e., one of these:

The IR filter glass itself came from a particular screw-in type IR filter; namely a 46mm Hoya R72 as shown here:

What I discovered is that the glass element in the 46mm screw-in Hoya filter is just a little bit smaller than the size of its threads (like most are, I suspect), something like 43mm if I remember correctly. And that size turns out to be just right to nest inside the internally-threaded portion of a Series VI retaining ring! You can remove the glass from the Hoya ring easily by just carefully prying out the snap-ring; the glass simply pops right out, nothing else holding it in. In my case since I had a couple extra Series VI retaining rings, it was an easy choice to turn one of them into a dedicated Series-VI IR filter by just adding the Hoya glass. Now all you have to do is place the glass in the retaining ring, and then find a way to hold it in place. For this it occured to me to try the snap-ring from the Hoya assembly, and it turns out that it actually does fit pretty nicely in the threads of the retaining ring. Unfortunately, the glass is tall enough that the snap ring doesn't get quite enough "bite" on the threads of the retainer so it is not really very secure (it can pop off too easily) So what I did was to seat the snap ring as best I could and then stake it in place with some super glue. So far this seems to be holding nicely, and now I have a good modern IR filter in "Series-VI" mount, which I can use on any of my Series-compatible cameras.

A couple other notes:
- As an alternative method of securing the glass, you could simply pot it directly onto the retainer rather than bothering with the snap ring. This would be a good permanent solution, but I wanted to be able to pull the glass out easily again if I ever needed to. I also think it looks more "finished" this way.
- Another thing is that my original intention was to find an IR filter glass whose outside diameter was just right to work directly as a drop-in Series-VI filter - i.e., something close to the nominal 41.3mm diameter of Series-VI filters. Unfortunately it is hard to find exact dimensions of the glass in commercial filters. But my guess is that something close to a 43mm filter would be about right, so you want to see if you can find that size. That would be a more elegant and versatile solution than what I have done.
- I can't guarantee the exact glass size of any particular filter - all I know is that the Hoya 46mm filter had the right sized glass. It is also possible that a "thin-line" filter might be thin enough to allow the original snap ring to be used to hold it in place in the retainer. That would be neat.

- Before getting an honest-to-goodness R72 filter, I also played around with making IR filters out of E6 slide film, and made a couple of these into Series-VI filters. Here is a picture showing two attempts at that to give you some other ideas. The first attempt (on the left) is in a cardboard or plastic mount (I forget which), while for the newer version (right) I was able to mount the film directly into the metal ring of an actual Series-VI filter from which I had removed the broken glass:

E6 IR filters.jpg