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[
     Button["Click Here", Print[10!], ImageSize -> {100, 100}]]
    
    AlignmentTest[
     Manipulate[Plot[Sin[x (1 + a x)], {x, 0, 6}], {a, 0, 2}, 
      ContentSize -> {500, 500}]]

    AlignmentTest[
     Grid[{{"\[Times]", "\[Times]", "\[Times]"}, {"\[Times]", "\[Times]", 
        "\[Times]"}}, Frame -> All, ItemSize -> {10, 10}]]
    
    AlignmentTest[Overlay[{Graphics[{Disk[]}], Slider2D[]}, All, 2]]