type
  extrec = record
    value: extended;
    dummy: word;
  end;

var
  f: file of extrec;
  e: extrec;

begin
   ...
   e.value := 1.0:
   e.dummy := 0;
   write(f,extrec);
   ...
end.

Since extended is not an official IEEE format (like you said), it's
possible that Visual C has a special type (like TP) that maps to
extended, otherwise it's probably a "long double".

    1. read the 'long double' into a 10-byte buffer.
    2. use a short ASM code snippet to convert it to a 'double'.

You lose some precision, but this is the best that can be done in MSVC.

The ASM snippet follows:

void ldbl2dbl(const void *ldbl, double *dbl)
{
    __asm   mov     eax,ldbl
    __asm   fld     tbyte ptr [eax]
    __asm   mov     eax,dbl
    __asm   fstp    qword ptr [eax]