Calculating/estimating volatiles based on other representative samples
If you are sure that the samples for which you have H$_2$O data are from the same intrusion than yours, then I don't see a problem for using these values for viscosity modelling, as long as it is clearly mentioned in the paper. After all, a sample is supposed to representative of the rock unit, and science is done by standing on the shoulders of giants!
If you have several historical samples with different H$_2$O contents, this gives you a range of water contents. Do you intend to use a model like Giordano et al. (2008) ? If so, you also need a temperature estimate, which you may have by geothermometry. These temperature estimates have an uncertainty, see Putirka (2008) for a review. For example, if you use the two-feldspar thermometer, it gives you $T$ to $\pm$30 °C.
So, what you could do is to model two scenarios: one &quot;cool and dry&quot;, with the lowest $T$ and H$_2$O content, which gives you the highest viscosity; and one &quot;hot and wet&quot;, with the highest $T$ and H$_2$O content, which gives you the lowest viscosity. This way you can be quite confident that the viscosity was somewhere between these two values. Usually a $\pm$30 °C interval will change your result by half a log unit, and a $\pm$0.2 wt% H$_2$O interval will change it by one log unit, so you'd get something like $\log \eta = X \pm 0.75$ Pa s, which is not that bad.