For audio, in particular, an alternative is to tap into the audio circuitry which may be elsewhere. Even the loudspeaker outputs can be used but then without additional switching, you cannot disable the internal speakers when you are using your stereo system.
Depending on the model of TV, doing any of this may be trivial to impossible - or a serious safety hazard.
A service manual or Sams' Photofact for the set will probably even identify the additional circuitry present in the higher priced models with A/V inputs.
If, on the other hand, everything is crammed onto a single circuit board with no evidence of A/V signals, it may be very difficult as suitable tap-in points may simply not be available.
If what you are really after is replacing a dead tuner/IF with your own tuner or converter, this may be possible but, again, may not be worth the trouble. The antenna isolation circuitry is probably external to the TV's tuner so yours could be substituted in its place. Of course, any user contact with the transplanted device would then have to be TOTALLY prevented since a serious shock hazard would be present for all metal parts and connections including shield grounds. In addition, many components would likely blow the instant power was applied if this were not done perfectly.
Unless you intend to always use the direct A/V inputs and forgo the tuner, you will need some way of selecting between them - a switch or relay. This could be manual - you push a button or flip a switch - or automatic. There are all kinds of ways to doing the detection - mechanical, checking for a low impedance connection, looking for a signal, using a switch, etc.
You will need a schematic - don't attempt this without one (for safety, if no other reason). Even some TVs that have A/V jacks use a live chassis and provide isolation for each signal (though this is the exception rather than the rule). So, even adding another A/V jacks to one of these would be hazardous!