Well, in a word, no.
Shikomizue is best thought of as a particular application of some systems, rather than a certain skill set itself. When it comes to Zatoichi, well, you're essentially looking at a reverse grip approach to sword, and that itself is a very rare thing to find as a classical area of study (in fact, I've never seen it... doesn't mean it doesn't exist, just that I've never seen it). Let's look at why...
If we are looking at the Japanese sword, without going too much into it's history and development, the Japanese sword (katana) was designed to be used with two hands (early tachi were lighter, and designed to be used one-handed on horseback, but that will still remove the use we'll discuss in a moment). Earlier in their history, the length of the sword was not regulated, so it could be as long as you wanted. Regulations came in during the Tokugawa Shogunate (17th Century onwards), standardising the length of regular katana to 2 shaku 3 sun, refered to as josun or teisun (roughly 70cm). A very few schools who used the longer blades managed to continue to teach the longer blades in secret, such as the Kage Ryu (which is honestly just an excuse to post this link...
), but the majority moved to the shorter blades.
As most sword systems were designed to use these weapons with two hands, especially if you are looking at an old armour based system, the use of the sword in a reverse grip would be rather unlikely. So if it was to come about, it would need to be a more "street" style, rather than battlefield (before anyone asks, by "street" here, I am simply refering to using shorter blades when not wearing armour, again circa Tokugawa Shogunate). In fact, the only "reverse grip" sword techniques I have come across are "specialty" iai techniques from systems such as Togakure Ryu, and Tenshin Shoden Katori Shinto Ryu. Within the Ninjutsu-related systems, Shikomizue is taught as a variation to the Hanbojutsu (three foot staff, or half staff).
So we have a weapon designed to get it's power from the use of both hands, and uses the advantage of it's length. To use it reverse grip removes both of these advatanges of the weapon (although it does give it a little more maneouvrability in close quarters), so it won't be a major aspect of any system. The history of the weapon also points away from such uses. With Zatoichi, you really must remember that the use of Shikomizue and the reverse grip approach is primarily a theatrical device to seperate him from the "bad guys" and other swordsmen who he is depicted with.