Inspired by recent plug-in frameworks that repurpose frozen layers from large language models (LLMs) as inductive priors, we explore whether such mechanisms can be extended to clinical time-series prediction without textual inputs or LLM fine-tuning. We introduce a lightweight plug-in architecture that inserts a single frozen LLM Transformer layer between an aggregated time-series representation and the prediction head. Unlike prior work focused on vision or language tasks, our study targets clinical time-series data, where LLMs typically underperform when applied directly.Experiments on two ICU prediction tasks from MIMIC-III show that the proposed plug-in exhibits heterogeneous effects across different backbones and tasks, with occasional performance improvements and minimal computational overhead. We further compare general-purpose and medical-domain LLM layers under an identical plug-in setting, analyzing how domain specialization interacts with clinical time-series models. Overall, our results highlight important limitations of frozen LLM plug-ins and motivate future work on understanding the conditions under which such layers may be beneficial.