Change author font for specific cite command

A possible solution is to introduce a toggle to determine whether we are in a text cite or not. Then the modification of \mkbibnamelast is parametrised to the toggle. This solution uses the xpatch package to modify the textcite bibmacro, to set the toggle to true just before executing the textcite macro and to false right after the textcite macro has been executed.

\newtoggle{textcite}

\AtBeginDocument{
  \renewcommand*{\mkbibnamelast}[1]{%
    \iftoggle{textcite}{#1}{\textsc{#1}}}
}

\xpretobibmacro{textcite}{\toggletrue{textcite}}{}{}
\xapptobibmacro{textcite}{\togglefalse{textcite}}{}{}