How to mix cream to increase its fat percentage?

There's only one thing that's typically sold with a high enough fat content that you'd be able to mix it into other dairy to get 48-50% fat ... butter.

I've found a page that says it can be done, but they're using a 'cream maker' ... a bit of searching seems to keep coming up with the name 'Royston Bel' or simply 'Bel' and attachments for a Kenwood mixer. I managed to find a patent from 1945, but it has only external diagrams, nothing internal.

I think it is quite difficult to increase the fat content of the cream with your proposed ingredients.

• Philedelphia cream cheese has about 35% fat.
• Mascarpone cheese is made from curdled cream. It has a higher fat content than cream due to the removal of whey. There can be about 44g fat in 100g mascarpone. The real fat content might differ. The only thing that I know for sure is that mascarpone has a fat content of 80% in the dry product.
• Crème fraîche is just soured cream and has the same fat content as cream - 35% (see also the wiki article in Dutch).
• Quark (full fat) in Germany has 52 - 68% fat but I think the regular quark has less than 10%.

Have you seen double cream / crème double (40% - 60% fat) in a grocery store? It's just cream with higher fat content. This could suit to your ice cream.

EDIT: I found an older question here on cooking.SE: Is there any way to make single cream to double cream? and Could I add butter to single cream to make whipped/double cream?

I'm wondering if it's possible to increase the fat content of cream if the cream is drained in a cheesecloth...

No, it's not possible to make high percentage cream from mixing normal cream. Not even with a high-speed blender. (Yes, I've tried). I don't know about Joe's device, but I'd be very skeptical about it. You might somehow get the fat inside, but I don't think it will behave like high percentage cream.

So, you have three cases

• You really need the high percentage cream. Typical case: the recipe requires to whip it. Sorry, but then you are out of luck. There is nothing you can do, short of buying non-homogenized milk (you'll have to go straight to the farm for that) and churning your own.
• What matters is the ratio. Your ice cream is one of these cases. There, you can use butter, but not any of the other products you mentioned, because they contain lots of other solids, which will throw the ratio off. You could find tricks for emulsifying the butter into the milk, but frankly, there is no need to bother. Just combine the butter with the other ingredients at some other time, not together with the milk. In a custard, you can mix it in after the custard has cooled. Creamed butter will make it especially fluffy, it will go like making buttercream. Calculate the proportion with a simple ratio formula, using 35% for your cream fat content and 83% for the butter fat content.
• The ratio doesn't matter, you don't really need heavy cream, but just need something richer or just a different taste or texture. There, you can use anything you want, in any proportion. Just mix away. And while you will be able to mix some of the things you listed with the cream, there isn't much need to do that. It is probably again simpler to add it together with the rest, instead of premixing.

Run your whipping cream through a couple of coffee-filters.

That should drain-off some of the water and leave you with more milkfat. It will take 2 hours or more for 2 cups to filter through, so you'll need to set it up in your refrigerator. As the comments have pointed out - SOME of the fat will filter through the paper-filter, so the end ratios of fat will be hard to estimate. It is going to take some experimentation on your part to make your recipe(s) to work right with your end-results.

Here's the relevant Alton Brown instructions: http://www.foodnetwork.com/recipes/alton-brown/clotted-cream-recipe.html

Cream makers did exist, and work. I think I still have mine, a Bel, somewhere. I used it a lot in the 70s, to make cream from milk and melted butter. I was poor, and cream was relatively more expensive then. You melt the butter in the milk, then pump it through a tiny aperture. This causes the milk and butter to emulsify or homogenise (i.e. you cause the oil and water elements to mix). My local milkman delivered un-homogenised Jersey milk and I also had a bent aluminium pipe, with a hole part way down the longer leg. You dropped this into a milk bottle and it automatically siphoned off the cream, stopping when the milk level reached the hole in the pipe.