Hi Eamon,
Your mentioned 6-bit Digital Attenuators, HMC624ALP4E, the HMC542BLP4E and the HMC792ALP4E, are all potential replacement candidates for the PE4312MSLBA-Z, with very comparable performances in most parameters except for the power handling performances at low frequency. These are GaAs-based attenuators, the power handling, Power compression and IIP3 will degrade at operating frequency below 200MHz.
Since our Dattns are not drop in replacement of the PE4312, we can find a best fit Dattn for your application. I'll be able to make some recommendations based on your conditions and requirements; such as freq range, input power, preferred control mode (serial or parallel), control voltage levels, Vdd level and etc. You can send me the information via a private message if you like.
Best Regards,
MN