Using a regulated, LINEAR power supply to float charge a lead acid battery (flooded or AGM) is a perfectly acceptable technique. Don't use a switching power supply to do that, because you risk damage to the switching regulator. In fact, the manual for my switching power supply specifically warns NOT to connect a battery to the output for exactly that reason. I keep a 10A linear supply around just for float charging batteries.
But as Mark K5LXP mentioned, you will cook your battery, unless you limit the time you do float charging. Float charge the battery for a couple of hours once a week maybe. But when the current into the battery drops to near zero, stop float charging. Unless you have an automatic device to do that, like with a Battery Tender.
Totally agree with AA4PB in that using a battery in place of a filter cap in a linear power supply is a bad idea.
Using a West Mountain PowerGate to connect both a battery and a power supply to a radio is a much better solution than just using a diode, because the PowerGate has a much lower forward voltage drop than the diode. The forward voltage drop won't be a problem for the power supply. But it might be for the battery backup, especially as the battery voltage starts to drop as it discharges. Plus, you need a seriously big honkin' diode to handle the current draw from a 100W rig, and you may need to be concerned with heat dissipation in the diode as well.