R xtable(): How to print text before caption?

Question

I would like to create a Latex table in R with the xtable package, which prints some Latex command (in this case \small\sf) between \begin{table} and the caption, like this:

\begin{table}
\small\sf\centering
\caption{This is a table}
\begin{tabular}{l}
Some table \\
\end{tabular}
\end{table}

I looked into the arguments of print.xtable(), but could not find anything helpful. Does anyone know whether there is a way to achieve that?

Here is a MWE:

set.seed(230)
DF <- data.frame(a = rnorm(5), b = rnorm(5), c = rnorm(5))

print.xtable(xtable(DF, caption = "This is a table"), caption.placement = "top")

Which gives:

% latex table generated in R 4.2.1 by xtable 1.8-4 package
% Thu Dec  7 15:58:13 2023
   \begin{table}[ht]
   \centering
   \caption{This is a table} 
   \begin{tabular}{rrrr}
     \hline
     & a & b & c \\ 
     \hline
  1 & -0.23 & 0.04 & 1.34 \\ 
  2 & 0.10 & 0.57 & -1.62 \\ 
  3 & 0.33 & -0.14 & 0.83 \\ 
  4 & 0.36 & -0.75 & 0.20 \\ 
  5 & 0.44 & 0.13 & -0.49 \\ 
   \hline
\end{tabular}
\end{table}