Okay, somehow managed to solve my problem thanks to both of you. Adam appears to be using an older version of MATLAB, but combined with Orion's solution it gave me the idea that led to me fixing my problem.
It's simply
xlab=zeros(rc,1);
for k=1:rc
xlab(k)=96+k;
end
set(gca,'XTickLabel',char(xlab))
After the image has already been displayed. rc is the number of rows (or columns, since it's a square board).
p.s. With just this, if the board is too big the tick labels will get all wonky. Something like
if rc>11
set(gca,'XTick',500*(1:rc))
end
might be required.