ummm.... it's not documented but the RGB command is also an ARGB command
i.e. rgb(255,255,255,255)
So to adjust the alpha of a2text you use
A2text Verdana(16),0,50,"Text",rgb(alpha,255,0,0)
example code
sync on : sync rate 60 : backdrop on : color backdrop 0
dim Verdana(48) as integer
for i=2 to array count(verdana(0)) step 2
Verdana(i)=a2CreateFont("verdana",i,a2Size_Char(),a2Style_Bold())
Next i
direction as boolean
alpha as integer
do
if direction=0 then inc alpha,5
if direction=1 then dec alpha,5
if alpha=>255 then direction=1
if alpha=<0 then direction=0
A2text Verdana(16),0,0,"Solid Text",rgb(255,0,0)
A2text Verdana(16),0,50,"Pulsing Text",rgb(alpha,255,0,0)
A2text Verdana(16),0,100,"Transparency: "+str$(alpha)+" - Direction: "+str$(direction),rgb(255,255,255)
sync
loop