I don't think the 897 can short those pins without the typical keying "interface". Of course you could use your foot switch, but that does not seem to be the best way to do it.
The FT-897D amplifier keying circuit is rated +50V and 400mA.
The L-4B definitely falls well below 50V, but I'm not sure about the 400mA. It would be very easy to measure this by just placing a VOM (on a 0-1A range) across the amplifier VOX keying pins to key the amp. Doing this should key the amp and at the same time read how much current the relay draws.
If it's <400mA, no need to do anything except connect the rig to the amp via its keying line; if it's >400mA, then a lower current keying interface is required.