OK, tried this over lunch - here's an example as a fluid file, seems to work for me at any rate... You'll need to provide a SVG image called "test_img.svg" but other than that should just work.

fluid -c test1.fl

fltk-config --use-images --compile test1.cxx

====================== test1.fl ===

# data file for the Fltk User Interface Designer (fluid)
version 1.0400
header_name {.h}
code_name {.cxx}
decl {\#include <FL/Fl_SVG_Image.H>} {public global
}

decl {\#include <FL/Fl_Box.H>} {public global
}

data svg3 {private local filename {../img/test_img.svg} textmode
}

class svg_rsz {open : {public Fl_Box}
} {
  Function {svg_rsz(Fl_SVG_Image *svg2, int X, int Y, int W, int H, const char* L=0) : Fl_Box(X,Y,W,H,L)} {
    comment Constructor open
  } {
    code {svg = svg2;
svg->resize(w(),h());
image(svg);} {}
  }
  Function {resize(int x, int y, int w, int h)} {
    comment {A resize method} open return_type void
  } {
    code {Fl_Box::resize(x,y,w,h);
svg->resize(this->w(),this->h());} {}
  }
  decl {Fl_SVG_Image *svg;} {private local
  }
}

Function {} {open
} {
  code {Fl_SVG_Image *svg2 = new Fl_SVG_Image(NULL, svg3);} {}
  code {svg_rsz *boxA = NULL;} {selected
  }
  Fl_Window main_win {open
    private xywh {882 285 249 221} type Double resizable
    code0 {boxA = new svg_rsz(svg2, 15, 15, 180, 180);} visible
  } {}
}

===== end ====