Can a 30-amp load with dedicated in-line fuses be safely connected to a spare fuse-box terminal on a 2001 Ford Ranger?

What you propose would not generally be advisable. The reason is that what you intend is construed as an add-on, for which the fuse panel harness is not equipped. If you take into account the wire sizing which has been kept to a minimum for carrying the anticipated loads for which the system is designed, then any added draw upon the circuitry would need to be kept at a minimum to stay within limits. It would be more prudent to equip the load you propose with a relay which served as a switch for your componentry, and powered the pull-in windings of that through the panel. Such an arrangement would draw far less than the load of the devices you power, causing less of a load upon the original wiring and fuse panel. The power feed for this could be, as you say, provided with an in-line fuse to the relay for the purpose of power interruption should something happen to the feed circuitry. Your outputs would be the same as you originally intended, except for being run through the relay. That's why, and how, things like your horn are run through without the need for bigger fuses. Nobody needs a fire in the dash caused by an overloaded power feed and under-sized wiring.