Fixing the position of math accents when using unicode-math in lualatex with a text font for letters and digits

You can set custom top_accent values by adding a font feature and patching the font table, but by default this will be ignored because you aren't loading a math font. So additionally, you'll have to declare your font a mathfont. Then, it still won't work because LuaTeX considers the font a legacy mathfont and therefore doesn't apply the OpenType mathfont based top_accent value. So you have to emulate being an OpenType mathfont. This requires you to set at least one math parameter. That's a bit problematic becuase this will overwrite the corresponding parameters from your primary mathfont, so you'll have to reload you primary mathfont later to fix up the parameters again.

A possible approach would be (comments inline)

documentclass{article}
usepackage{unicode-math}

% First some catcode setup, not really important
begingroup
  longdefx#1{directlua{unexpanded{#1}}}
  catcode`#=12 catcode`%=12
  expandafterendgroupx{
  --[[Declare a helper DeclareTopAccent to invoke the package later]]
  local id = luatexbase.new_luafunction'DeclareTopAccent'
  local mappings = {}
  token.set_lua('DeclareTopAccent', id)
  lua.get_functions_table()[id] = function()
    --[[This is executed when the command is called. We have to parse the input. Take a peek at the usage of DeclareTopAccent below before trying to read the code, then it should be relativly easy to follow]]
    local t = {}
    repeat
      local cp = assert(token.scan_int(), 'No codepoint found')
      token.scan_keyword'='
      t[cp] = assert(token.scan_int(), 'No offset found')
    until not token.scan_keyword';'
    assert(token.scan_token().cmdname == 'relax', 'Final delimiter missing')
    --[[Save the parsed mapping in a global table and then send the index back to TeX]]
    mappings[#mappings+1] = t
    tex.sprint(string.format("top_accents_id=%i", #mappings))
  end

  --[[Now implement the feature. Nothing particularly interesting here, it's 
    the same as almost any use of otf.register: Take the feature value, do some lookups, apply to characters]]
  fonts.constructors.features.otf.register {
    name = 'top_accents_id',
    description = 'Change selected top_accent values',
    initializers = {
      base = function(tfmdata, value, features)
        local mapping = assert(mappings[value], "I'm going to strike")
        local characters = tfmdata.characters
        for cp, top_accent in next, mapping do
          --[[For some reason some properties have different names here than the native LuaTeX name. E.g. top_accent is just accent]]
          assert(characters[cp], 'Why are you doing this to me?').accent = top_accent
        end
        --[[For top_accent to have any effect, LuaTeX must consider this a modern Math font.
          To fake that, we'll provide a dummy parameter if necessary. MinConnectorOverlap = 0 should be relativly safe.
          Remember to reset your math parameters after loading this font]]
        if not tfmdata.mathparameters or not next(tfmdata.mathparameters) then
          tfmdata.mathparameters = { MinConnectorOverlap = 0 }
        end
      end,
    },
  }
}

setmainfont{Source Serif Pro}
setmathfont{Erewhon Math}[Scale=1.1]
% Example of usage: DeclareTopAccent expects pairs of codepoint and top_accent value,
% separated by ; and terminated with relax.
% The unit of top_accent is milli-em, so in most cases values around 500 are a good starting point.
setmathfont[range=it, RawFeature={DeclareTopAccent `W=550;`V=450relax}]{Source Serif Pro Italic}
% Reset Math parameters messed up by the other fonts
setmathfont{Erewhon Math}[Scale=1.1, range={}]

begin{document}
$hat V$
$hat W$
end{document}

But while this is possible, it is a lot of work to adapt these for all characters and you still don't get a font as good as a proper math font. You should really try to switch to an OpenType math font as soon as possible.