Results are known in many different cases to bound powers of L-functions on average over a wide enough family. I am interested in results for general number fields, not only for the rationals, for powers of the quadratic twists. More precisely let $\pi$ be an automorphic representation of $GL_n$ and $\chi_d$ the quadratic character associated to $\mathbb{Q}(\sqrt d)/\mathbb{Q}$. Are there any bound known of the form $$\sum_{d < D} |L(1/2, \pi \otimes \chi_d)|^2 \ll D^{\theta + \varepsilon}?$$
All the results I find deal with the rational field, I would like to know if there is any problem in generalizing to general fields.