Setting manual thermal voltage for a diode in LTSPICE
You can set the temperature for each component individually using the following approach:

After placing your component and selecting the correct model for it, do a CTRL+Rightclick on the component. In the newly opened window, search for the "SpiceLine" field and enter temp=500 (or whatever value).

Now your diode will have that temperature where the rest of your circuit does not.

Example:



In the LT-Spice netlist the line for the diode is displayed like this:

D2 N002 0 1N914 temp=500


With following result: