Hope someone knows of a plugin for Motion that will do this for you.
There are several text filters available for After Effects that can do that. Sorry, I do not know if any of them can be run inside Motion, doubt it.
Text anarchy from Digital Anarchy comes to mind and I think the major text packages from Boris include a cursor option in may of the type-on effects.
It's not really that hard to fake it, tho, once the text is golden. We used to do it in AE often. The trick, after you set keyframes for position, is to make it blink in time with the unseen keyboard operator's text entry. Tedious, but easy.