How could I efficiently implement this "TTL load"?
The load is in fact equivalent to single TTL input. TTL family has open emitter inputs. That Base-emitter junction is horizontal diode in picture. 3 diodes in series are base-collector of NPN transistor followed by 2 base-emitter junctions cascaded. As The Phantom noted, you can use simple TTL buffer input as load. 

You can see typical internal circuit for TTL gate on page  9.

ftp://apollo.ssl.berkeley.edu/pub/cinema/04.%20Science/TTL%20Cookbook_0672210355.pdf