G8HAJ has designed a standalone MT Hellschreiber interface that uses a small 14 pin 16F1615 PIC device.  It can be used either to send a beacon message or inreal time QSO mode using RS232 interface from a terminal.

The speed / bandwidth is made continuously variable from a few 100Hz down to fractions of a Hz using a pot to control the settings.  Which makes it useable at LF

The downside is that it uses a hybrid SMT/CMT which he calls Pol yPhase or PP/MT-Hell type of modulation that does require a linear transmitter - so no high power with a switch mode Tx   

More details and a full description and PIC firmware download at Graham's web site  http://g8haj.uk/    or specifically   http://g8haj.uk/?page_id=288

Four of us using that module recently had a simultaneous full-quadplex (4 way version of duplex :-)  QSO on the QO100 satellite.    A few screen dumps of the QSO underway showing all four stations can be seen here :