From what I've found we don't have a way of bringing this up via code, we only receive events - http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.viewmanagement.inputpane.aspx It looks like metro expects us to use native input controls and as such it will control the visibility of the soft keyboard. IE has this working however so maybe there is a way to do this.
If under the hood this is using the text services framework, we might be able to solve this via nsTextStore support.
This is latched in via UIA.
The basics for this support landed on elm yesterday. Advanced text selection via touch and the UIA selection interfaces will happen in bug 743690. http://hg.mozilla.org/projects/elm/rev/072177ac7736