Human operators in the upcoming Industry 4.0 workplace will face accelerating job demands such as elevated cognitive complexity. Unobtrusive objective measures of mental workload (MWL) are therefore in high demand as indicated by both theory and practice. This pilot study explored the wearability and external validity of pupillometry, a MWL measure robustly validated in laboratory settings and now deployable in work settings demanding operator mobility. In an ecologically valid work environment, 21 participants performed two manual assemblies - one of low and one of high complexity - while wearing eye-tracking glasses for pupil size measurement. Results revealed that the device was perceived as fairly wearable in terms of physical and mental comfort. In terms of validity, no significant differences in mean pupil size were found between the assemblies even though subjective mental workload differed significantly. Exploratory analyses on the pupil size when attending to the assembly instructions only, were inconclusive. The present work suggests that current lab-based procedures might not be adequate yet for in-the-field mobile pupillometry. From a broader perspective, these findings also invite a more nuanced view on the current validity of lab-validated physiological MWL-measures when applied in real-life settings. We therefore conclude with some key insights for future development of mobile pupillometry.