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Resolved observations of the stellar halos of nearby galaxies yield critical insight into the mass assembly history in these systems comma allowing the predictions of cosmological models to be directly confronted with experimental data. Andromeda (M31) is a key location for this endeavour comma offering a second data point comma and exhibiting strikingly different halo properties comma to the Milky Way. Here we propose to take the first step towards obtaining full 6D orbital phase-space information for stellar substructures in the M31 halo comma by undertaking a pilot survey to measure proper motions for a small set of globular clusters with first-epoch ACS-WFC imaging taken 11-17 years ago. By comparison with recent HST proper motion studies of larger stellar systems out to the distance of M31 comma we expect to achieve a precision of virgul0.010-0.015 mas-yr with these baselines. This compares favourably with the observed 1D (line-of-sight) kinematics of M31.s outer globular cluster system comma which exhibit a velocity dispersion of virgul100 km-s (0.027 mas-yr) plus coherent rotation of comparable amplitude. These pilot measurements will provide the first 6D phase-space information for any extragalactic stellar stream comma and enable a new comma independent estimate of the shape and mass of the M31 dark halo at very large radii (50-120 kpc). By demonstrating feasibility and optimising our methodology with this data set comma we ultimately aim to extend similar measurements to a much larger cluster sample in future comma facilitating detailed orbital modelling that will accurately reconstruct the main events in M31.s accretion history.