This command is probably not possible, it would be a ton of effort for minimal gain. However, there is good news! The current system already supports what you want to do. There is a key thing you are missing about bard songs and the melody system. While you are limited to two beneficial songs total, the limit is two detrimental songs *per target* not two total.
So your example above is already quite possible with our current system. You can absolutely cast mez on one mob, slow on another and then mez a third. Using the single target slow line will give you a bit of extra time on the slow. Selo's Consonant Chain is five ticks instead of the normal three, and Requiem of Time (62) is ten ticks. The PBAoE slow is also a good choice, as you can slow up to six mobs without dropping your targets. This would be Largo's Melodic Binding and later Selo's Assonant Strane. This will keep your mezzed mobs slowed as well. Keep in mind that /s 5 is your best friend as well as setting a key near your spell keys to toggle last two targets. This can be done in the Options Menu, on the Keybinds tab.
If you have more than two mez targets you need to cycle through them in order by clicking rather than using the toggle target key. Make sure you click on the head of the NPC rather than the body. Do not use new model elementals as they are notoriously hard to select by clicking. One mez target in addition to the target you are killing is no sweat. As Anvyl pointed out, simply let your mez song continue without interruption and attack another target. If you wish, you can then sing up to two detrimentals on this second target before redoing the mez on your first. Two mez targets is easy also unless you are trying to stack another song like Occlusion to reduce resists. The maximum amount of songs you can sqeeze in before the first wears off is four, but this can vary based on when it lands. That is because the tick is server side, not client side. So your song can land anywhere in that cycle. So a maximum of three mez targets (or song casts) is safe.
Another thing that you might want to practice as soon as level 42 is chant kiting. Instead of swarm kiting, which doesn't work here, bards will use this method above to keep two dots running on two different mobs. You can round up a large group and just use target nearest to select your next two. Mez can help to break up the group, or snare, but be aware using snare will trigger summoning mobs to use that ability. All of these techniques will depend on your charisma and the mob's resist rate. A very handy AA you will get later is Jayla's Insight. This will allow you to check not only resists but also level on a mob. This information is important to other players and you can make use of it yourself so you know which mez to load and if your charm will land. Good luck!
edit: Here is
a handy guide on the wiki to explain AE and chant kiting in more detail.
