You can take advantage of the undocumented fact that commonly RGB triples can instead be RGB quads with the last value being alpha value.
However! Setting the alpha of lines this way is not permanent. It does not survive copying objects; it does not survive saving and restoring; it might not survive any automatic redraws (for example due to rescaling the axes, including because of adding more tiles.)
fig = figure();
ax1 = axes(fig);
plot(ax1, rand(10,15))
ax2 = axes(fig, 'position', [.5 .5 .3 .3], 'xcolor', [0 0 0 .3], 'ycolor', [0 0 0 .3], 'color', [1 1 1 .5])
