Apple does not allow SMS nor Phone replacement apps in the App Store, so this will never be possible without jailbreaking. Since you indicated you would prefer not to jailbreak, you will not be able to do this.
The closest you will be able to get would be to use a third-party SMS service which may either cost or perhaps force you into using a different phone number for SMS specifically with that app. You will need to review third-party apps available to see if they provide a custom keyboard to do what you wish to do.
Long story short: Built-in? Not possible outside of jmlumpkin's suggestion.
I believe that the siri command for imessage is: Message Alice i'll be there soon. This will not prevent it from sending a text message. To prevent it from sending a text message, go into settings --> messages --> and turn off the "Send as SMS" switch.
In her contact it should look like this:
Alice Appleseed
Phone: (XXX) XXX-XXXX
Email: aliceappleseed@xxxxxxx.net
What you need to do is try to message her on one of these accounts.
Try saying: "message alice on her email...I'll see you later"
Then: "message alice on her mobile...I'll see you later"
The point is: you want to state where you want to send the message. If you just say "message Alice", then siri will ask you where you want the message to go.
Since the iPad doesn't have a phone number, you message it through its email address (this will have to send an iMessage). The non-iPhone uses its phone number (this will have to send a text message).
I hope this helps.
Update: Brought to my attention by the author of the original question, the method above: "message alice on her email...I'll see you later" is not fully effective.
Rather, saying it this way:
"message alice on iCloud...I'll see you later"
will send the imessage to Alice's iCloud email address (which must be set up by her as an E-mail address which can contact her via imessage). Note: If I'm correct, the E-mail account used as a contact point does not have to be an iCloud account (although it may be better to use one anyway). For instance, you could use the E-mail account of johnny.appleseed@example.com, yet label it as an iCloud account in Contacts, and it SHOULD work the same.
Sadly, I have since stopped using Siri, simply because I have been too busy, so feel free to correct me if I am wrong about this all.
Glad I could help. (surprised that you remembered my answer, since it was written a good while ago.)
Best Answer
As Apple doesn't allow for third-party keyboards outside of apps, you'd have to find (or make) an app that does what you want and design your own keyboard to do that.