Hello Sergio,
The netif_add calls the function tivaif_init from the porting file. Inside the function tivaif_init the function call is as follows
psNetif->output = etharp_output;
psNetif->linkoutput = tivaif_transmit;
If i check the tivaif_transmit function, "tivaif_transmit(struct netif *psNetif, struct pbuf *p)" it maps the pbuf to the MAC controller for transmission. It is this buffer that needs to be modified for LLDP frame. However the call stack is always from TCP or DHCP (even when ARP is called which is a link layer packet).
Regards