How can I measure the pH of high-viscosity fluids like dough?
In addition to buckminst’s answer, for weak acids, which are not fully ionized, the pH of the solution depends on the ratio of the concentrations of the conjugate base $\ce{A-}$ to the neutral acid $\ce{HA}$, especially in the pH regime close to the $\mathrm{p}K_\text{a}$ of the acid (the buffer region). Diluting the solution will change the concentrations, but in the same way. The ratio will not change. The Henderson-Hasselbalch equation can be derived from the mass action expression for $K_\text{a}$ of a weak acid:

$$\ce{HA &lt;=&gt;H+ + A-}$$
$$K_\text{a}=\dfrac{\ce{[H+][A^{-}]}}{\ce{[HA]}}$$
$$\text{p}K_\text{a}=-\log{K_\text{a}}=-\log \left(\dfrac{\ce{[H+][A^{-}]}}{\ce{[HA]}}\right)=-\log[\ce{H+}]-\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$
$$\text{p}K_\text{a}=\text{pH}-\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$
$$\text{pH}=\text{p}K_\text{a}+\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$

If the volume increases by a factor of 10, then $[\ce{A-}]$ and $[\ce{HA}]$ both decrease by a factor of 10, and $\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}$ remains constant. 

Since the acids in dough are likely acetic or lactic from fermentation; carbonic or tartaric from leavening; or citric or ascorbic from preservatives – all of which are weak acids – diluting the dough might not change the pH.