Can a feedthrough capacitor be considered (and used) as a capacitor with a very low equivalent series inductance?
No, they can't, but not for the reason you think. You can get simple feedthrough bypass capacitors, as well as the feedthrough filters that Tom Carpenter talks about in his answer.
If you wanted to take a line from one box to another, and isolate the RF by using a shunt bypass capacitor, you would need a capacitor of very low series inductance. In fact, for most applications, to do it in a single stage would require an unphysically small series inductance. If you crank up SPICE and see what you need, you might find you need well sub-nH, which you just cannot achieve with a 2 terminal shunt capacitor. 1mm of wire is, give or take, about 1nH.
But a feedthrough cap is not a 2 terminal device, it has three terminals. The problem with a 2 terminal device is the incoming RF generates a voltage across the stray inductance of both connections. This voltage is then output to the next box.
With a 3 terminal feedthrough, the outgoing voltage is picked off from the capacitor in series with only the ground connection. The output voltage therefore includes the voltage across the ground lead stray inductance but not the input lead stray inductance. As the ground lead is very wide (several mm) and very short (<1mm) in a cyclindrical feedthrough, this inductance is well sub-nH.
Although it works well in isolating RF when used as a 3 terminal device, the fact it has only one low inductance lead means it cannot be used as a low inductance 2 terminal capacitor.
The improvement over a 2 terminal shunt capacitor is much the same as you get when measuring a low resistance with a 4 terminal Kelvin connection, you do not need leads of extremely low resistance, as the voltage is picked off on the resistor itself.
You can also buy 'Filtercon' filters which look very much like feedthrough capcitors, where the input and output lead inductances are enhanced by adding a ferrite bead to each, to turn the simple shunt capacitor into a 3rd order low pass filter. But these are more expensive than a feedthrough capacitor.
In a related part, have a look at the BAR81 from Infineon. This is a PIN diode designed for shunt operation. One lead is made very low inductance, by use of the leadframe two pins in parallel, this goes to ground. The other lead is a hop-on, hop-off, connection, the same way that a feedthrough capacitor is.
Simply, no. Feed-through caps intentionally have a high inductance on the signal path to allow them to work as filters. By running the current through a thin and quite inductive trace within the capacitor you end up with an L-C filter which is makes them so good at filtering and why you can replace ferrite beads with them.
Essentially a feed-through cap resembles the left hand diagram below, as compared to a regular parallel capacitor on the right.
You can get low ESL feed-through caps, however this is referring to the parasitic inductance of the ground path (the circled inductor in the diagram above). You want to keep the parasitic inductance of the capacitor part of the circuit as low as possible to ensure good filtering.
For low ESL in general you want to look at capacitors specifically designed for this. Once such option is those in reverse SMD packages - 0306, 0508 etc. For these sorts of packages, the inductance is reduced by making the connection to the plates wider.
I tested this using a network analyzer some time ago, using Murata NFM21PS106 SMD feedthrough caps.
The gist of it is that, when you consider ESL from one terminal only, they behave more or less like a SMD capacitor of equivalent size. Most of the ESL is in the connection from your power plane to the cap anyway, so the type of cap matters little. Reverse geometry caps have lower ESL, but that's half due to the fact you can stick more via's on them.
X2Y caps, though, have very low ESL, because they form a clever low-inductance system with the vias. You cannot neglect via and mounting inductance, it is essential.
As filters, though, feedthrough caps are ridiculously good.