Reread the question. It's Scorching Ray, for which you cannot hold the charge.
I know what the question is. I was simply answering to you, so as to clarify that while holding a charge all the melee-type actions are entirely possible and covered by RAW.
Not that you said otherwise, I'm just making the differentiation between attacking with the charge and attacking while casting the spell.
And, actually, melee touch spells do not give you a touch attack as part of the standard action to cast the spell. Rather, they allow you to make a single touch attack in the same round in which you cast the spell as a free action.
I forgot about that clarification, still what you say is not entirely true.
I understand that the spell's description says: "Casting time: 1 standard action", (one which is still subject to AoOs) still there is nowhere written that the touch attack is the "free action" of the process.
The entire process is a standard action, a quite unique one actually that allows you to split the two sub-actions, and let's you combine them as you wish with the move action.
Touch Spells in Combat
Many spells have a range of touch. To use these spells, you cast the spell and then touch the subject, either in the same round or any time later. In the same round that you cast the spell, you may also touch (or attempt to touch) the target. You may take your move before casting the spell, after touching the target, or between casting the spell and touching the target. You can automatically touch one friend or use the spell on yourself, but to touch an opponent, you must succeed on an attack roll.
IMO, again this only shows how minor the casting time of those spells is. I cannot think of the touch attack as being the "minor action" (it's a normal attack, without penalties, that happens during one's round). I 'd probably consider the casting as being the "minor action" of the entire process.
Look at it this way:
The wizard speaks a word (minor sub-standard action), and his hand suddenly flashes with magical energy etc etc... Then he moves close to the enemy (move action) and attacks (major sub-standard action) while taking all the time and the effort to make that single attack. Honestly, I do not see those sub actions being the other way around.
This is why I take it that it is possible to do a CDG with a touch attack spell. The whole process just turns from a standard action into a full round action. (In that case, splitting the action is not possible, one has to be standing next to the victim, only 5' are allowed before and after)
Again this might not be clear by RAW, but I totally see it as being possible within the concept of those rules.