What is the technical term for ARINC 429 data injected into a heartbeat stream going to an LRU?
ARINC 429 is both an electrical bus standard and a data standard.

The electrical standard defines a unidirectional, one to many (Tx to Rx) serial data bus that is transmitted using bipolar Return to Zero (RtZ). This makes the signal self clocking. If no data is being transmitted, the line stays at 0 VDC (Null) which consequently removes the clock as well.

Since there is only one transmitter on a line, there isn't any "injection" of data onto the bus by another source.

The ARINC 429 Data standard uses the serial bus to transmit packets of information, called labels. These labels are 4 bytes (32 bits) long.

An ARINC 429 receiver is designed to expect a constant stream of labels to update the receiver's internal data within a certain time limit. If a label isn't received within the specified period it is considered stale and is left to the receiving LRU to decide how to handle stale data.