If we write the regression equation like so:
y = B x + C
But if both sides are standardized, then we have:
(y - my)/sy = B ( x - mx )/sx + C
Now solving for y by multiplying by sy then adding my:
y = B ((x - mx) sy)/sx + C sy + my
y = B sy/sx + C sy + my - (B sy mx)/sx
Therefore the metric coefficients are:
B' = B sy/sx
C' = C sy + my - sum((B sy mx)/sx)
In the above notation, the multiplication operator is implied.
R test code:
coef(m)[1]*sd(d0$y)+mean(d0$y)-
(coef(m)['x1']*sd(d0$y)*mean(d0[['x1']])/sd(d0[['x1']]) +
coef(m)['x2']*sd(d0$y)*mean(d0[['x2']])/sd(d0[['x2']]))
Gives 1 as desired.