Hi,
I'm thinking of trying silicone to seal the cap and I'm wondering if anyone else has tried this, or has a technique to seal the cap to keep the nib wet.
Short answer: no.
If you are referring to a kit pen, there are two cap arrangements to consider. A snap-cap is almost always loose, and there really isn't a way to seal it. If the kit features a screw cap, the threads are press-fit into the brass tube, and that arrangement should be pretty tight.
If you are referring to kitless designs, then you are probably (but not necessarily) using some kind of plastic (aka 'resin') for the body, and it will be reasonably tight. And kitless designs usually feature a threaded cap.
I have lots of fountain pens - some commercially made, and some shop-made. And the experience I have had with them drying out is all over the map. I have an El Grande (kit) that seems to stay wet forever. Likewise a plastic Lamy Safari, and also a no-name metal pen. I also have some kit pens (both CSUSA and PSI kits) with snap caps, and a no-name commercial metal pen that will dry out if I don't use them for four or five days. And I have a very expensive Mont Blanc that would dry out if I didn't use it a few days - which is one of the many reasons I no longer use it.
The solution is to use your pens frequently. And if you are using several pens simultaneously, set up a rotation scheme so that you use a different pen every day.