We introduce the concept of Space-Time Super- Modulation according to which additional low rate and highly reliable information can be transmitted by further super-modulating blocks of traditionally modulated and space-time encoded information. This is achieved by exploiting the redundant information introduced by the space-time block codes and, specifically, by efficiently mapping transmission patterns to specific information content. It is shown that Space-Time Super-Modulation can be efficiently used in the context of machine-type communications to enable joint medium access and rateless data transmission while minimizing or even eliminating the need for transmitting preamble sequences. Compared with traditional approaches that use encoded preambles or preambles based on Zadoff-Chu sequences to transmit the signature information of transmitted packets, Space-Time Super-Modulation can achieve throughput gains of more than 35% when transmitting blocks of 200 symbols.