/cast is considered secure stuff, so what you want is not possible.
With particular reference to your example, there is no real reason to use a lower rank spell anymore, because all lower ranked spell now costs the same amount of mana as the max rank spell. (Yes go check, even my rank 1 conjure food costs the same as my max rank.)
In some cases, lower ranks actually cost more than the highest ranks. For example, the shaman spell Chain Heal:
Rank 1, obtained at level 40, costs 24% of base mana.
Rank 2, obtained at level 46, costs 24% of base mana.
Rank 3, obtained at level 54, costs 24% of base mana.
Rank 4, obtained at level 61, costs 24% of base mana.
Rank 5, obtained at level 68, costs 19% of base mana.
You see the same thing with the druid spell Healing Touch, which is first learned at level 1. Ranks 1-12 cost 38% of base mana. Rank 13, obtained at level 69, costs 33% of base mana. Again with mage Fireball: ranks 1-13 cost 25% of base mana, while rank 14 costs 21% of base mana. Warlock Shadow Bolt: ranks 1-10 cost 23%, rank 11 costs 17%.
In-game the cost is shown as actual mana, but the mana cost of all spells is now a percent of your base mana. Your base mana is how much mana you have without the bonuses from gear or buffs. I'm not entirely sure whether or not mana granted from talents (such as the shaman talent Ancestral Knowledge, which grants 10% additional intellect, and thus additional mana) are considered part of base mana or not.