ive been using this for ages.
Even though there frontend sucked ***, and they are no longer a company they did make a nice OSK and the only one i know that works in this manner.
To activate the keyboard you only need to click into any area that will accept text (text boxes) and a small button will popup next to your cursor.
It also will zoom the area that is receiving the text input or not.
The only thing i dont like about it is its not skinnable