Were there any plans or proposals for "unilateral geoengineering deployments" before this paper was publsihed?
I think the introduction of the paper itself covers this adequately:

… a growing number of studies have investigated regional SG application scenarios, which could prove preferential to a global application by restricting the geospatial magnitude of the climate response or by being used to target specific climate changes8,9,10,11.

The cited references are:


Robock, A., Oman, L. &amp; Stenchikov, G. L. Regional climate responses to geoengineering with Tropical and Arctic SO2 injections. J. Geophys. Res. 113, D16101 (2008).

MacCracken, M. C. On the possible use of geoengineering to moderate specific climate change impacts. Environ. Res. Lett. 4, 045107,  https://doi.org/10.1088/1748-9326/4/4/045107   (2009).

MacMartin, D. G., Keith, D. W., Kravitz, B. &amp; Caldeira, K. Management of trade-offs in geoengineering through optimal choice of non-uniform radiative forcing. Nat. Clim. Change 3, 365–368 (2013).

Haywood, J. M., Jones, A., Bellouin, N. &amp; Stephenson, D. Asymmetric forcing from stratospheric aerosol impacts Sahelian rainfall. Nat. Clim. Change 3, 660–665 (2013).



Of those, [9] seems the most obviously relevant, although it covers more techniques than just SAI. One particularly relevant passage from [9] reads:

… materials lofted this high and staying
in the atmosphere for extended periods tend to spread out to
cover the globe. As a result, both their climatic influences
and any unintended side effects (e.g., increased scattering
of incoming solar radiation that diminishes the efficiency of
direct-beam solar energy technologies) tend to be global in
extent.

Since [9] is focused on discussing regionally-targeted geoengineering techniques, the presumed global spread of SAI is here being mentioned mainly as a potential disadvantage.
I think Jones et al. are actually raising two concerns with their work:

Unilaterally deployed SIA targeted at a particular region could have negative side-effects. This is the topic most relevant to your question. As the references show, there were indeed plans for this, at least in the sense that it had been discussed in the literature for some years. I don't know of any state drawing up concrete plans for SIA stations yet, but it's probably a good idea to raise these concerns before the policy train gets rolling.

SIA might be deployed at a single site as a globally targeted SG technique, with the assumption (as seen in [9]) that it doesn't matter where you do it because the aerosols will spread evenly over the globe in any case. Jones et al. show that this will not necessarily happen.