Unidirectional PHY transceiver
Do you know if there are some chips that are capable of establishing a unidirectionnal connection over a RJ45 cable?


At least any IEEE 802.3-compatible 100BASE-TX-enabled PHY could be programmed via the MII Management Interface (MDIO/MDC) to support one-way connection. The target configuration is as the following:


Disable Energy Efficient Ethernet (mandatory if supported)
Disable Auto-Negotiation (mandatory) 
Force Full Duplex (mandatory)
Force 100 Mbps Mode (mandatory)
Force "permanent" Link Up (implementation-specific) 
Disable Auto-Crossover (Auto-MDI/MDIX, implementation-specific)
Disable Far-End-Fault Generation (implementation-specific)
Disable (any) Power Save mode(s)/feature(s) (implementation-specific)
Disable (any) Cable Diagnostic feature(s) (implementation-specific)
Disable (all) other smart feature(s) (implementation-specific)   


Here "mandatory" means that the feature is defined by the Standard, "implementation-specific" means that the feature may be, if presents, specific to the particular choice.

One additional trick for the transmitting side PHY if all the above (suddenly) has no enough effect: connect the RX± pins of the PHY to the TX± pins of the PHY - before or after the transformer is case-dependent. If the PHY has no internal termination you have a chance to terminate such a "local loop" only once. There is no need in such the trick for the receiving side PHY of a planned uni-directional connection.


  Is it possible to reprogram an existing chip?


Yes, via MDIO/MDC if the chip you use is IEEE 802.3-compatible. The most (if not all) chips are.