And I may not know the output impedance of the transmitter either, though I do know it's important that looking into the filter, the transmitter sees 50 ohms. It's unclear to me how this requirement is satisfied when performing the filter design.
The input impedance of the filter is typically specified so as to match the output impedance of the signal source. This allows maximum power transfer from the source generator.
The input impedance of the filter circuit can be validated by doing series/parallel circuit analysis starting from the load resistor and working backward toward the source. The voltage of the source should be shorted out for this analysis leaving the source resistance as the last circuit element to be factored into the series/parallel analysis.
As mentioned by Kevin Ried AG6YO, R1 and V1 in your schematic represent the signal source with R1 as the output impedance of the source. R1 does not waste output power of the signal source, but only represents the source impedance. In amateur radio, the transmitter output power is specified into a specific load impedance, typically 50 ohms. This is not necessarily the output impedance of the transmitter as the transmitter's output impedance is time variant. But it is the impedance at which the amplifier will develop its specified output so this is an acceptable proxy for output impedance of the signal source and the desired input impedance of the filter in this case.
And for that matter, is characterizing the load impedance as a resistor really accurate? For example, a half-wave dipole may have a known resistive impedance at resonance, but if the filter is intended to suppress spurious emissions then it must work at other frequencies where the dipole will present a potentially very different impedance. What can be said about the performance of the filter under these conditions? Will it still effectively suppress harmonics?
You are correct to question the validity of the filter characteristics when connected to a load impedance that varies with frequency, such as a typical antenna where reactance is added in series with the feedpoint resistance as the frequency moves away from resonance. The frequency response of the filter must be analyzed at each frequency in question with the appropriate complex load at that frequency. This will typically result in a different characteristic curve than found with a simple resistive load. The affect of this change can generally be minimized adding an additional filter pole.
A load that changes its complex impedance with frequency also presents the conundrum that the signal source (a transmitter, for example), does not see the desired impedance looking into the filter. That is to say that the input impedance of the filter is dependent on the impedance at the output of the filter. So the changing impedance of the antenna with a change in frequency changes the load impedance of the filter which changes the input impedance of the filter which is the load the transmitter sees. This results in a reduction in power of the transmitter which is typically accompanied by a change in harmonic content. So to avoid this multi-variable situation in lab tests or simulation, it is best to analyze each in isolation and mathematically combine the results.