While I'm not sure how you'd go about it, it seems like something you might be able to do via a spellcast rule, much like how my scholar spellcast will change the spell being cast (i.e.: stratagems on the wrong light/dark arts) to the appropriate one if I mistype or its not ready.
A snippet example of what I mean:
Code
<elseif spell="Penury|Parsimony">
<if BuffActive="Light Arts|Addendum: White">
<action type="ChangeSpell" Spell="Penury" />
<action type="Command">input /recast Penury</action>
</if>
<elseif BuffActive="Dark Arts|Addendum: Black">
<action type="ChangeSpell" Spell="Parsimony" />
<action type="Command">input /recast Parsimony</action>
</elseif>
</elseif>
Course you'd need the spellcast plugin, a more or less fully xml file, and would have to figure out if there is a condition to check to see if the spell you intended to cast can be casted at the time (i.e.: if Spell="Slow II" Recast="0" , or something like RecastGT="0" to make it change to Slow if the recast time for Slow II is greater than zero seconds)
The rules guide for spellcast are here:
http://wiki.windower.net/plugins/spellcast/rules
and the rule might look something like this (assuming it'd even work)
Code
<if spell="Slow II" recastgt="0">
<action type="ChangeSpell" Spell="Slow" />
<action type="Command">input /recast "Slow II"</action>
<!--The above so you know when Slow II is ready-->
</if>