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It seems that the handling of the Alignment-option is not consistent for all functions using it. Panel for instance seems to support numeric values for this option

Manipulate[
Panel["\[Times]", ImageSize -> {100, 50}, Alignment -> {x, y}],
{x, -1, 1},
{y, -1, 1}
]


while with Grid this is not supported. Knowing this, you could check interesting function with something simple like

SetAttributes[AlignmentTest, HoldAll];
AlignmentTest[func_] := Row[{
Manipulate[
Append[func, Alignment -> {x, y}],
{x, -1, 1},
{y, -1, 1}
],
Manipulate[
Append[func, Alignment -> {x, y}],
{x, {Left, Center, Right}},
{y, {Bottom, Center, Top}}
]
}]


You see that in many cases numeric values for the Alignment options can be used

AlignmentTest[