I don't understand sparring. You take a technique that's designed to drop your attacker, and you want to practice it on your fellow deshi. So, you change the technique, make it safer to practice on your partner, and then square off on the mat. Now you're attacking and defending, executing techniques that only resemble the technique you wanted to use in the first place, and are still running the risk of injury.
See, the trouble with sparring is that it's not what the techniques were designed for. Take shomen-ate for instance (a technique used in Tomiki Aikido wherein the tori [nage] locks uke's spine backward by use of a hand to the face, thus effecting a throw backward). At speed, this is not a technique that I would recommend in a sparring situation, because of the long history of injuries from doing just that. In order to make it safe enough for sparring, I'd want to move the hand from uke's chin to uke's chest. Then, I think, we could spar.
Trouble is, now you're doing a different technique. It's no longer shomen-ate, it's something else. "Shomen" means front of the head, not somewhere on the chest.
It's difficult to spar with techniques that are designed to be dangerous to the reciever, without changing the techniques for safety.
It is for this reason that I would recommend the use of randori and an aikido version of kakari geiko for training instead of 'sparring'. Either way, there will simply have to be techniques that you don't get to practice on your fellow deshi.
In kakari geiko, we go for only a few minutes at a time. The longest I've ever seen anyone do kakari geiko is 3 minutes. Uke attacks, and although uke is still compliant, tori (nage) doesn't know in advance what the attack will be. Once the technique is done, uke recovers and attacks again without pause, with a different attack. The idea is for tori to react appropriately, without prior arrangement. In this way, even some of the more dangerous techniques can be thrown if they're done right.
~J