Your question says there is 1000ft between the TA and TL, but depending on how QNH compares to QNE from day to day and from place to place, the actual thickness of the layer will vary; it will always be at least 1000ft (though rarely more than 1999ft). This guarantees that an aircraft at the TL and an aircraft at the TA will always be properly separated.
Flght levels (QNE) below the transition level (TL) could be less than 1000ft above other aircraft below the layer. Therefore, they are invalid.
Altitudes (QNH) above the transition altitude (TA) could be less than 1000ft below other aiircraft above the layer. Therefore, they are invalid.
Ergo, within the layer itself, there is neither a valid flight level nor a valid altitude; there is literally nothing in between for ATC to assign. You can only climb or descend through it to a valid assignment on the other side.
If you did try cruising at an invalid altitude/FL within the layer, then you would likely not be properly separated from crossing aircraft either at the TA, at the TL, or both. It's better for everyone to not have to worry about such things, which is why the layer exists in the first place.