Ok, big edit to the question as people have suggested.
I would like to use the master transmogrifist prestige class all the way to L10, as I find it interesting to use two of its key abilities. Polymoprhing as a reaction, and the level 10 ability, where you can add on one feature from another creature when you polymorph.
However, a simple build based on taking wizard and MT means that the earliest one can get access to MT 10 and shapechange is level 21 as the MT loses 4 spellcasting levels. This means that a generalist wizard build will be considerably better at transforming into creatures at levels 17-20, as the advantages of shapechange over polymorph (25 HD limit instead of 15, supernatural abilities, extra duration, and ability to change multiple times and more) outweigh the MT's abilities.
I am looking for ways to make this build more workable, particularly at levels 17-20. The original question revolved around getting access to shapechange earlier than level 21, and this is certainly a good option. However, any other suggestions people have that are capable of making this a viable build pre epic are good too.
The aim is to make a polymorphing specialist that can change into creatures better than a wizard of the same level can. This polymorphing ability will then be used to turn into a creature or combination of creatures that has abilities to counter the current situation. General qualities that I expect to be 'tacking on' using the MT level 10 ability would be elemental resistances/immunities, spell resistance, damage reductions (and the means to overcome them), flight (taking a speed slightly faster than my opponents speed and the best available maneuverability for that speed), Fast healing. If no specific counter is obvious for the fight then I will likely do something boring - take 8 tentacles as the book suggests, give a dragon good dex or give good natural armour to something that lacks it.
Using these abilities as a reaction is designed such that my defenses are always available, as long as I know something about what is attacking me.
Combined with finding it enjoyable to play as monsters and enjoying the roleplaying aspect of turning into creatures as a way of swiss army knifing, this is why I've requested the use of MT10 as part of this build.The point of optimising is because the default build doesn't give me the enjoyment of playing the character I want until level 21, whereas I would like a RaW way of playing a character with these abilities at lower levels, ideally 17.
This making any sense to anyone yet?